There's this Thing that y'all don't seem to get.
Gryffindors don’t give a shit about rules. The most hardline of them don’t even care about people. They care about justice. Right or wrong, black or white, there are no shades of grey. If it’s just, it’s always just; if it’s unjust, it’s always wrong. Hermione’s ruthlessness makes her a Gryffindor. She is absolutely sure that she is on the side of justice in everything that she does, and it’s such a Gryffindor trait.
Because Slytherins are ruthless, but they care about rules. Their own rules, usually, but rules nonetheless. They will impose parameters and limitations on themselves just so they have a framework to operate within. If doing something means violating their own internal code, then they’re not gonna do it. Even fucking Voldemort is like that. He broke every single fucking rule the Wizarding World ever put in place, but damn if he’d break his own.