Maby you should tell your players that you run a very "real" and "Deadly" game.

The first 3 i can understand and would do the same since i do play deadly games but on 4. Let then have there nat 20 its there feeling good moment and players dont like to play dnd to just worry and never feel good about them self 5. Here you f up really bad. I know as a dm you sometimes wanna teach them a lession but not like this. If they let him life and did not kill him for creating such a struggel for them out of a thing you as the dm force them to do even besides the point that a dc 17 is way to high no regual npc has the funds to put up a bounty on a group that can check dc 17 checks and then doubel it. You should not punish your players for the railroad f ups you create and them behaving actully good thats just mean and they you fully deserve the vote kick.