Answer: Punching them in the midriff, doing a pulling takedown from their arm, punching them into the kidneys from the back (a bit halfheartedly. I wasn't trying to break anything, just to ensure they stay down for half a minute or so). And then running away.
Considering he was trying to rob me with a knife in the middle of the night, I feel somewhat justified. I did feel bad afterwards, but he was caught by the cops later that night (thanks to my tip) and from what I heard from them after I had given my version of the events the next day, I caused no serious physical injuries; half-an-hour with a doctor and some non-prescription painkillers was all he needed. Clear cut case of self-defence, the cops said. And indeed, no one came to follow up on that or anything.
Beyond that, I have no violent impulses beyond the 'I want to shake some sense into him/her' kind. Perhaps it's due to my regular martial arts training: whatever aggression I have gets resolved sparring or on technique practices.