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.
This is actually remarkably true, at least as far as the Gryffindor one goes. The people I’ve just met kind of blink at me when i tell them I’m a Gryffindor and go “You would literally murder someone in cold blood if you thought they were going to hurt your people.” And for some reason this baffles them. Nerve and chivalry are well and good, but it’s the vicious, all-consuming determination to follow honor at the cost of all else that makes a Gryffindor, as far as I’m concerned.