Serial Killers

I found it interesting that while studying serial killers and psychopaths, they all blamed someone else. Not a single one of them ever managed to take responsibility for their crimes. I found this odd, because I remember learning about how they feel good when killing people, so it surprises me that they would not take the credit when offered, even when they knew they would be in prison for the rest of their life. Was it a spur of the moment thing for them to take credit? Or did they actually believe that being taunted by a girl in the 4th grade was the reason they had murdered and raped 50 women?