For this situation, given that 5e has a specific rule for melee attacks to result in a K.O., the DM called this incorrectly. They made a realistic call…people accidentally kill each other in fights far more often that movies care to showcase. It is why fighting is both high risk and ill advised in life. But the game has the K.O. rule there in the name of fun, and it should be respected.

I see where they are coming from though in terms if an action backfiring, but for OTHER situation. This may not be it, but there is absolutely a place for “your succeeded, but not the way you wanted.” That should be known at the table though because not everyone expects a game to run that way. In order for it to work well, the insight skill becomes pretty important.

Case in point, the intimidation skill. An NPC might have a fight or flight response to being bullied or intimidated. Lets say the PC is trying to scare and NPC into fleeing a potential confrontat. A very high “success” roll just means you intimidated them and they feel intimidated.

An NPC with a fight responce may see the PC as a high level threat and attempt to squash them on the spot.

An NPC with a flight response might just run away screaming or appologizing for any offense they may have cause the PC.

Regardless of the players aim, certain NPCs might have a

A GM might be running a game where a high number means getting as close as possible to the outcome you desire.

A GM may consequently being running a game where where a high number means getting the most impactful.