This movie accurately depicts Dahmer as a person. It's a little rough around the edges but when you understand the film you will really respect it.
Dahmer was insane and denied an insanity plea because he was under the influence of alcohol. I don't believe he should have been put in prison with regular criminals.
He was homosexual but was embarrassed of it so he started off by slipping roofies in drinks and having sex with homosexual guys after they passed out. This doesn't equal insanity. There have been hundreds and thousands of cases of guys doing this to girls and they were called "horny", not insane.
But many of those horny guys eventually had trouble at their workplaces. If they didn't get sexually aroused by a lot of female co-workers, they easily could have clashed with the ladies over mundane issues such as how to maintain "TPS reports." The movie Office Space eloquently depicts that mundaneness. Even without arousal, horny men approach TPS reports in different ways than women do. Jeffrey Dahmer's job history didn't seem to include much experience with TPS reports.