You're wrong, people can be predisposed to schizophrenia, dementia, antisocial personality disorder, borderline personality disorder, depression..you name it...there is even substantial evidence that the tendency for mass murder is a genetically inherited trait...
Antisocial personality disorder, otherwise known as a sociopath, and my bet as to what this murderer is, is a genetic trait...one that makes the afflicted person pretty much incapable of feeling anything for another human being...he might kill someone, he might not, but either way it probably won't be because of what anyone did to him...and he won't feel any guilt about it one way or the other.