My players tried to capture one of my bosses (they didn't know it was a boss) and after a few rolls they succeeded. They started to interrogate him (the NPC is a human) and after obtaining the information they wanted he was stabbed in the chest. Basically, imagine having one of the most powerful warriors of the kingdom killed by a throat cut because the rogue rolled 19 on Stealth.
Now here's the thing: how do I handle realism and one-shotting bosses out of combat? Because I can think of excuses but I don't want my players to feel discouraged or think that their actions are meaningless if the DM wants this NPC to fight them.