A) Believe the guy is really innocent
B) Just want/need the money
C) Believe that guilty or not, as representation is a right it is a patriotic duty to make sure that right is seen to
Now, as for specifically serial killers, given the large number of cases where they had the wrong guy several times (Sharon Tate Case, Bundy Killings, Pedro Alonso Lopez's killings etc) it seems like about a 30% chance by the time the guy gets to trial that he is innocent.
The problem is that there is a ton of pressure to solve that kind of crime, and it can lead to shit being rushed. Given that chance, wouldn't you want to be sure that the right guy is getting punished? In order to do that each side needs representation, otherwise we cannot be certain what the truth is (really even then, but its about as close as we can get)