Any blunt object > knife

One good hit to the head. It's over.

Getting stabbed feels like getting punched, most people don't even realise they've been stabbed.

It's why whenever someone uses a knife in self defence they end up in prison.

Hard to explain why you needed to stab someone 20 times in self defence but that's what it takes.

There a lot of innocent things you could carry in your pocket that turn a hammer fist into a lethal blow.