You've got the trolls (it may just be one guy, honestly) who will find someone in a thread and out of nowhere start screaming at them, trying to start a fight.
You've got someone like him who pretty obviously follows one person around for a day or two, calling names and hurling insults.
There's the legitimately mentally ill who are just by their nature difficult to be around for any length of time.
There are multiple people on here who have something going on with them -- booze, personality disorder, pain meds, who knows -- and they will lose their complete and entire shit out of nowhere because they misread something or got a little confused.
Then there are the old conservatives who are always in a pissy mood.
There's the alt-right/rightwing trolls who are always as cranky and morose as possible. Their toxic personalities are compounded by the fact that several of them have created fake DL personas that they use to gain trust and positive recognition, but then someone you'll know as a "nice lady who loves cats" will suddenly post "gay men are responsible for allowing trannies to literally murder real women in jails and bathrooms and I don't think I'll ever forgive them, it's best if they die out" (that's an actual example) and that starts to wear on people. They think, hey, this person gave me a lovely bread recipe, they said my cat was cute, now they're telling me I'm responsible for the deaths of all women and they want me dead?
Found a group of these alt-right interlopers on one of their little hatefest threads yesterday where they were so miserable they were literally talking about offing themselves because SJWs exist. One of them even posted a video of some sad song and said they'd see everyone on the other side.
Now, I'm an asshole, I think rightwing fuckheads being miserable is the height of hilarity, but I think most normal people on DL are probably overwhelmed by the constant stream of maudlin negativity, and because a majority of DLers either won't or don't understand how to use ignore in a way that would make their experience better, they just soak in negativity for the entire time they're here.
Of course, it follows that they start lashing out, too. Some days you can't even crack a joke without a half-dozen DLers screaming and frothing at you because they're so unhappy they didn't even recognize that it WAS a joke.
There's no solution to it unless someone starts moderating for real, rather than what happens here: no moderation for a couple years straight and then suddenly, in a fit of spite, banning a whole bunch of people including those who didn't actually do anything wrong, then "fixing" a technical problem by breaking it or turning it off.