Dennis Reynolds' mental illness is frequently hinted at and discussed over the course of the series.
It's unclear what is wrong with Dennis. Popular fan theories have labeled him a sufferer of antisocial and/or narcissistic personality disorder. On the show, he has been diagnosed with borderline personality disorder, although this does not rule out potentially comorbid disorders. Regardless of diagnosis, Dennis is not a well man, and this is shown time and time again on the show.
Symptoms of his mental illness include:
an extremely grandiose view of himself and his accomplishments
extreme sensitivity to rejection and/or perceived abandonment
persistent inner emptiness
frequent bouts of intense rage and other negative emotional outbursts (e.g. crying publicly), that get more frequent and severe as the series progresses.
complete lack of self awareness or understanding of decency
preoccupation with sexual and physical violence
alcohol, tobacco, and crack addiction
hallucination in response to stress
impulsive actions that cause harm to himself or others
a reliance on codependent relationships to meet his emotional needs
manipulation for personal gain
the desire to control everyone and everything in his life
Dennis routinely mentions sadistic acts, out of the ordinary emotional experiences, and grandiose claims about himself without any awareness that others might be disturbed by them:
Dennis often identifies himself as god-like, his appearance created by the gods, a "golden god" or as God himself.
While attempting to recreate a scenario to prove Mac is a serial killer, Dennis disguises himself as a painter to stalk and murder the Coffee Shop Waitress as she leaves work. When preparing to execute his murder, he was reminded by Dee that they really weren't going to kill the woman. Dennis, upon realizing Dee was correct, expressed his disappointment about having to scare her rather than actually kill her.
He offered to "frame bang" the Lawyer's wife in her sleep, and was surprised when the Lawyer referred to it as rape. He acts if he doesn't know what he's doing and when the lawyer asks him not to rape his wife, Dennis tries to act as if his intentions were pure but then cuts himself off saying "Fine, I won't do it". ("Paddy's Pub: Home of the Original Kitten Mittens")
Dennis has absolutely no qualms about manipulating women into sleeping with him by, among other things, calling them in a threatening voice, slashing their tires, and forging prescriptions for his (non-existent) grandmother. Then he "separates entirely" and moves on to his next conquest, not caring one bit about the fear, distress and heartbreak he has caused. ("The D.E.N.N.I.S. System")
Mac is disturbed when Dennis tells him that a good reason for them to have a boat is so they can get women out in international waters, where they will not refuse their sexual advances "because of the implication." He falsely assures Mac that he's not actually going to rape any women, it's just that the women will feel like they should give them what they want, "because of the implication". ("The Gang Buys a Boat")
He is disturbed by Mac's weight gain (as seen in Season 7), so he takes it upon himself to give Mac diet pills (which he describes as "Mexican ephedra") while telling him that they are "size pills." Mac's weight loss causes him great personal anguish, but Dennis doesn't care, because he has concluded that Mac must lose weight. ("The Gang Gets Analyzed")
After getting married to Maureen Ponderosa, he starts "having feelings again, like some kind of 14 year old kid". He is also confused by the idea that Mac has feelings all the time. ("Mac Fights Gay Marriage")
When visiting Dr. Jinx with Mac and Charlie, Dennis asks if the doctor has any sort of solution to cause him to have feelings again. ("Charlie's Mom Has Cancer")
At the Gang's high school reunion, Dennis in a fit of rage, claiming those who have crossed him will pay "the ultimate price", goes opens a secret compartment in his car's trunk to gather his "tools," which include zip ties, duct tape, a flashlight, a video camera, and various other instruments to subdue and restrain people. Though he claimed the items were for sexual bondage, the others identify him as a replacement for their unstable friend "Psycho Pete."
While walking with Charlie in the woods, Dennis refers to himself as a "pretty methodical person". Charlie responds with "Oh yeah, like a serial killer", which Dennis takes as a compliment. ("The Gang Gets Stranded in the Woods")
In season 8, he "gets off" on "the thrill of wearing another man's skin" by pretending to be Brian LeFevre.
In "Gun Fever Too: Still Hot", Gunther denies Dee and Dennis a gun after finding that Dee was institutionalized for lighting a girl on fire, and that Dennis has a record of sexual misconduct. In reply, Dennis tells Gunther he just made his "list."
In "The Gang Squashes Their Beefs", Dennis proposes an idea that the people they have wronged sign a peace treaty. When Frank asks why he always wants people to sign "Creepy documents" Dennis responds "Well Frank, once something's in writing that means it's set in stone. Then no one can do anything to stop me." Later in the episode Dennis locks all the gang's guests in the apartment after it has caught fire, by drilling the door shut.
Dennis is a generally cold hearted and cruel person and he bangs the waitress knowing Charlie is in love with her (and Dennis doesn't even like her) and having no sympathy for Charlie after how hurt and angry he was.
Dennis lacks remorse and very rarely shows any sort of empathy.
In "Psycho Pete Returns", Dennis tells Dee he is going to skin her and turn her skin into a lampshade or fashion her into a piece of high end luggage. He then goes on saying he could even add her onto his collection. When Dee asks him if he really has a collection of skin luggage, he replies "Of course not Dee, don't be ridiculous. Think of the smell. YOU HAVEN'T THOUGHT OF THE SMELL YOU BITCH!". He then goes on saying if she says another word, he'll cut her up into pieces and put those pieces into a glass box that he will display on his mantle. He later says he had a "rational conversation with a reasonable man" and is diagnosed with Borderline Personality Disorder.
Dennis is generally very manipulative (The Dennis System) and arrogant.
In "The Gang Exploits the Mortgage Crisis", After finding the family in the house, Dennis talks about a graphic twisted scenario they can do with the family but says they should just leave because it's the easiest.
In "How Mac got Fat", Mac tells the priest that Dennis always tells him "Don't let words like "no" stop you from getting what you want."
In "Chardee MacDennis 2: Electric Boogaloo" Dennis is molding his own vision of love in the form of a woman's head in a freezer “This is a woman’s head in a freezer, and it’s supposed to represent the preservation of love forever and ever!”
In "Charlie Catches a Leprechaun", Dennis orders a pair of girls that they have picked up (well, more like kidnapped) in the "Paddy Wagon" to "smile" for his website, despite the fact that they are terrified and crying. He tells them that it's "not supposed to be scary", even as their terror grows. When they tell him that they do not want to be on his "weird website", he tells them "If you don't comply with me, you're going to end up on the weird one." They let the girls out, but Dennis blames them for "not experiencing our business properly", implying that he thinks that what happened is their fault.
In "Wolf Cola: A Public Relations Nightmare," during an appearance on a morning news program to defend Wolf Cola, a dog who's in the studio to do another segment gets away from its handler and goes up to Dennis, who is very obviously angered and agitated by the experience. He talks about how much he hates dogs, and suggests that owning them as pets makes no more sense than eating them. The host is horrified by his reaction, and it turns public opinion, which they had swayed in their favour, away from Wolf Cola.
He frequently shows that he has unreasonable expectations of women and how the "ideal" woman should appear. In "The Aluminum Monster vs. Fatty Mcgoo" he becomes disgusted with a model for being "hugely fat" despite her being quite slim by all appearances.
“I'm not going to take no for an answer because I just refuse to do that, because I'm a winner and winners... we don't listen to words like 'no' or 'don't' or 'stop!' Those words are just not in our vocabulary.”
“I'll tell you what buddy, I can help you out. I'm gonna toss a frame-bang your way. Here's how that works: I slip into your house one night while your wife is sleeping... and I ease into her real nice. That way you're both cheating on each other and she can't clean you out.”
“What do you mean, what do we need a mattress for? Why do you think we just spent all that money on a boat? The whole purpose of buying the boat in the first place was to get the ladies all nice and tipsy topside so we can take them to a nice comfortable place below deck, and, you know, they can't refuse. Because of the implication.”
“Oh, uh, OK. You had me going there for the first part. The second half kind of threw me.”
“Dude, dude, think about it. She's out in the middle of nowhere with some dude she barely knows, she looks around, what does she see, nothing but open ocean. (Imitating female voice) "Oh, there's nowhere for me to run. What am I going to do? Say no? (shakes head)"”
“OK. That seems really dark.”
“Nah, it's not dark. You're misunderstanding, bro.”
“I think I am.”
“Yeah, you are. Because if the girl said no, the answer, obviously, is no. But the thing is she's not gonna say no. She would never say no. Because of the implication.”
“Look, look, this is about much more than just business. This is about the thrill of wearing another man's skin. Feeling his innermost wants and desires and being in control of his every single move. That's how you get off. Now don't you guys want to get off with me?”