Reading this again doesn't make the dude appear any less of a scumbag for stealing the kill of someone who was giving her absolute all to beat an opponent by herself. Not that the MC would really care. He's not the kind of person who would.
Normally I would say it would be disappointing if he could beat the dragon just like that, but who am I kidding? In the OP category of isekai the MCs are able to defeat anyone and anything no sweat. Except for the mighties of all monsters called Cockblock, which is unbeatable for more than 9 out of 10 isekai heroes.