@Nk9bjP4A You're assuming that the MC is some super human who can one man army any and all adventurers on the floor without issue. He is clearly approaching this as an infiltration grab and go, this isn't the time for a power fantasy murder fest, he likely doesn't even have the power for that. Equally importantly here, who was it that opened the portal for him? It was the freaking guild master herself, if there was a sudden mass murder of adventurers on the 10th floor, you think there would be no repercussions? Seriously? He would just me making MORE enemies.
@NazrinMaus
1. Yes, elf rescue from immediate harm is the only goal here. Killing the prince is an extremely bad idea as I described above and would only serve to put both of them (him and elf) in even deeper shit (remember, the elf is a political piece right now, and her being an adventurer is part of her agreement with the king). Remember, he is weak on his own and he has nowhere to go. His only goal in life is to delve into the dungeon, if he makes enemies with the guild then he personally is SUPER screwed.
2. Again, it is both a terrible idea and quite unreasonable for him to start a murder spree of innocent adventurers here.
3. Again, see above.
4. You make it seem like he had a long discussion here. His brief chat didn't take much time at all and skipping it wouldn't have changed anything for the reasons above.
Essentially in order to actually kill the prince he would have had to also kill a significant number of other innocent adventurers that happened to be on that floor. An act that is both extremely risky and would also likely make him an enemy of the guild that manages the dungeon itself. This would make it impossible for him to achieve his own personal goal of going to this world in the first place. Not to mention that the elf girl he just saved is clearly NOT ok with unnecessary killing, making the act even more unrealistic as it would put the elf in MORE danger, going against the whole point of why he is there in the first place.
He is not all powerful, the people around him are not idiots, the MC is also much smarter than 99% of isekia protags. Regardless of possible future complications this outcome was the most realistic one.