He is disgusting for sure, but his crimes seem very personal. Parents, brother, wife, hit man.
Did he know Valerie at all?
There's no indication that, if he did it, he even knew the house was Percy's house. If he did it, then he also broke into Pirrucello's house the morning prior. According to the wife, and police records, he was just violent and crazed, especially, according to her, after he had his brother killed.
Very true of him not being accountable. And it's easy to see how the local cops would dismiss many of his actions as domestic.
Either way, his brother was killed with a gun his wife purchased for the brother a few days prior, and under mysterious circumstances. At least one, and likely two men associated with him vanished from the face of the earth and his former neighbor, in a neighborhood he had returned to many times after moving away, ended up bludgeoned and stabbed to death.
And then of course, in the spring of the year Valerie was killed, he attempted to murder three people, including his parents, at their home around the corner from Percy's, only none of the three, nor the cops, knew he had killing them in mind. The plan unwound before it got that far.