bombotta almost seemed like he was trying not to have to kill conan

I do agree he didn't have anything personal against Conan and didn't seem too enthused with the prospect of having to kill him once the job was done(Although with Wilt's acting it's a little difficult to truly tell if that's what he was emoting...=P).

Though causing the cave-in wasn't any attempt at mercy or a way to avoid killing him. Taking out Conan and his party would be difficult, especially when he knew the princess would be against it too. So the cave-in was an "easy" way to trap them and allow the pursuing enemies to do the job. Plus it leaves their fate nebulous to the princess. Of course in the end that didn't work out so well.