Most of these questions have focused on the role of the DM in determining the expectations with the players around scenarios like this. But I do want to point out that there is a rule to help support your ruling in this situation.
In the DMG pg 273, there is an optional combat rule for Massive Damage which says that when half or more of a creatures HP is done from a single source of damage, the creature can roll a Con save (DC 15) and if failed potentially reduce the creature to 0 HP (among other randomly determined effects). Of course, if you are using this rule you would need to use it in all situations, PCs included, which again goes back to making sure outcomes like this are clear and up front with players.
But at least in this case, you have a rule to point to as to why a situation resolved the way it did.