Because all psychopaths don't end up being serial killers, doesn't mean that the majority of serial killers aren't psychopaths (in fact they are).

Does this mean all psychopaths are guilty immediately? No.

Does this mean that psychopathy is an issue that we should be concerned about and use it as a starting point to narrow down the more dangerous subset of the psychopathic population? Yes it does.

Learn to do reasoning using set theory - it will set you free.