You do realize that he was treated awfully by his father and that has an affect on his character

Also he was under the influence of the mind flayer when he kidnapped those people. so basically it possessed him

Like @Deku aka Izuku Midoriya said, Billy's character was more understandable because of his father's bullying, however that doesn't excuse his behaviour. And yes Mind Flayer control.

Though all scum baggery aside, Billy did warn Karen to leave him alone, which saved her life, and he tried that again with Heather but the Mind Flayer had taken over him entirely by that point.

Slightly unrelated, my head canon for Billy is that he's bicurious and had a crush on Steve earlier on in season 2. That obviously changed when he thought Steve was a creep and went berserk in him.

Yay so Billy was treated badly by his abused father but he loved his mother very much and thought that it was Max fault about it! But Billy was a bad person until in the end became a good person before he even died.

Billy was abused by his father, left his home for a town he considered a shithole, and was possessed by The Mind Flayer. Cut my man some slack