It’s very commendable that so many commenters are worried about hurting (or worse) their attackers.
I’ve never heard of any muggers, etc. worrying about what harm they may do to US.
Being that we are all members of a group that has been beaten, raped and killed by experts for the last 3,000plus years, I would say let’s learn to defend ourselves (from proper teachers who teach proper hashkafas along with the skills) and worry about OURSELVES FIRST. Once we are at that point, we can, if necessary, discuss further the other legal and ethical issues that go with having these skills.
To paraphrase General Patton… “It is no kiddush HaSh-m for a Jew to get beaten up, it’s a kiddush HaSh-m when a would-be Jew attacker learns that attacking a Jew was a BIG mistake!