This logical fallacy is called "ad hominem", which means "to the man" in latin. It means that, instead of debating about the specific question, you rather attack the person, in an attempt to discredit him and his words.
It is a logical fallacy because, when debating about anything, you should always and only talk about the actual point, instead of considering who said what.
If a serial killer claims that 2+2 equals 4, you can say something like "don't listen to him! He's a serial killer!". Yes, he may be one of the worst human being to have ever lived, but it doesn't change that this specific claim he made is true.