I'm not sure about that. There are reasons the rules exist.

There's an old saying that "he who fights monsters becomes a monster himself". L hasn't resorted to killing his targets, while Kira has... The conflict between them is about the morality of killing. Is it really alright to kill, even if the victim was a killer themself? It probably depends on what's going through Kira's head when he kills, which we probably won't ever know for sure.

The criminal was Sakota Hifumi.