I mean, I didn't warn the user at the time. I told him to be more careful next time.

Situations like these are iffy. RDM is out of roleplay, so preventing it would also be out of roleplay (and thus being RDM in and of itself), but the user gave as his defense that he'd possibly lose his Iridium if he didn't do anything. Considering the fact we don't refund for rulebreaks makes it kind of tricky whether or not you should be allowed to kill threatening users that shouldn't act threatening towards you.

So yeah, it's tricky, difficult and hard to explain - a punishment isn't needed.