Killer's motivation SPOILER ALERT
When Kreizler begins talking to the killer about his mother issues, the killer mockingly says"You think it's because of THAT?" leaving Kreizler to believe he was wrong.
However, some time before, as the killer is tormenting the captive boy, he seems to be mentally re-enacting a bath scene between he and his mother... He says stuff like "She only loved me when she bathed me" and "Pretty little bird in it's nest" (referring to boys genitals) "It's so nice I'll cut it right off". or words to that effect, which seem to be things his mother told him while bathing him.
So when the killer denied that his mother was the reason for the killings to Kreizler, it seems to me he was lying.