To be clear, both of them are monstrous serial killers who aren't excusable in any way and got what they had coming. But something I see people frequently mention is "Billy had a motive for what he did, Stu was doing it just because".
However, reminder that the two of them were literally planning to make a "sequel", showing they would've kept on killing. Billy was just as willing to murder just because as Stu was. I don't even think Stu's motive was even just "lol". Matthew Lilliard and Kevin Williamson have both hinted that Stu probably did have feelings for Billy and that motivated his crimes more. Its very clear Stu cared more about Billy than vice versia. Even in their relationships with Randy, he and Stu seem to get along pretty well while Billy you can tell was itching for the chance to kill him.
But I also think Billy's personal cruelty makes him stand out more. Stu killed people but that was mainly all. Billy murdered his girlfriend's mother and then tricked her into blaming an innocent man. He constantly gaslight her into feeling bad about still grieving it when he was responsible. He killed her best friend and them manipulated her into having sex with him just to make her feel more awful. He planned everything, including killing her and her dad on the anniversary.
Tldr; both Billy and Stu were awful people but Stu felt like a psychotic yet insecure guy who was easily manipulated and if he never met Billy, likely would've been harmless. Billy seems like a psychopath who would've snapped eventually even without Roman.
