This question introduced a long post on my Facebook page: "Why do some Bernie supporters and leftists insist on insulting Hillary?" My lightly edited response follows.
Attacks on Hillary Clinton's record and positions are legitimate, keeping pressure on her and her campaign to do what her critics want her to do: that is how this very serious game is played. To speak to this as a non-rhetorical question on insults, though — here, I think, are some of the broader reasons.
(1) Too few Americans spend a lot of time in actual communities, i.e., places where people are stuck with one another for the foreseeable future. This leads to the temptation to say any damn thing one feels strongly about.
(2) Where Americans do live in communities, many of us live in the results of "the big sort," i.e., surrounded by people with similar attitudes, so saying any damn thing we feel strongly about probably will bring agreement.
(3) Many of us spend time in virtual "communities," with "community" emphatically in quotation marks: places where we can be pretty anonymous and enter and leave at low cost, and places where we feel we can say any damn thing we feel strongly about.
(4) We're in another age of sentiment ("Get in touch with your feelings, Luke"), where it's seen as unproblematically good to feel strongly and share those feelings with the group.
(5) As the custom of "busting" ("signifying on," "cut contests," "doing the dozens") show, the adolescent male tradition of the flyting — insult contests — is alive and both well and ill and increasingly unisex. Insults are sincere and sincerity is good ... and again, people can say any damn thing, with extra points for "transgressivity," until they breach some still- or recently-fashionable taboo.
(6) Relatively few Americans grow up political and so haven't learned that there are instances when one must affront the neighbors and get into arguments, and therefore one should rigorously avoid pissing others off unnecessarily.
(7) Relatively few Americans grow up political and in real neighborhoods, so many fail to learn to never piss off the neighbors unnecessarily because you never know when you might need a favor.
(8) Many young Americans have had their self-esteem maintained strongly and work under the idea that they can say any damn thing and insult people and still be able to ask those people for favors.
(The editor of The Miami Student student newspaper sent me a Christmas card, which was a nice gesture, and used the opportunity to drop my column. She reassured me, though, that all was well since she'd replaced me with a colleague of mine who was a highly popular professor who she was sure would make an excellent columnist. My friends and I joked that she'd probably ask me for a letter of recommendation, and I said I might write her one because at a young age she had the sort of total sociopathic insensitivity typical of real-pro newspaper editors. Within a week, she asked me for a letter of recommendation.)
(9) The relatively hard Left and hard Right attract people who put ideas above people and put their ideas above just about any people they are not politically infatuated with. Such folk will say any damn thing they think demonstrates their love of the Good, the True, and the pretty ugly, but screw it: if you disagree, you're just wrong and evil.