@Reishin I know I am a few months late, but wanted to comment anyway.
Knocking people out doesn't really work like that. Generally speaking if someone is out for more than a few minutes you've likely killed them or cause permanent brain damage. He could have tied them up, but that risks the whole magic thing again. Even if he did knock him out and didn't tie him up the doc would be up and likely alert people in less time than it would take for him to escape. He also doesn't really have any moves for incapacitation, his training was focused on quick, efficient, killing moves. Based on the people reaction to him, the fact that he is in unknown lands, and had little in wyay of supplies/equipment I'd say that killing was the best option he had for making it out. It sucks, it is shitty for that doc since he probably wasn't a bad guy, but that is life. Sometimes you don't have the option of a good vs bad decision, sometimes it is just Shitty Option A vs Shitty Option B.
Also IDK if you are caught up on the manga or not, but there is actually a REALLY good reason he was taught the "Kill quick and first" mentality.