I'm going to Devil's Advocate on the part of the player.
Describing the 3 humanoids as "humanoids" is a lot different than saying something like "a hideous purple creature that walks upright like a man with a face like that of a spider". If the player heard "humanoid", he may have imagined actual humans, or orcs or elves. If the player thinks he's chasing three humans, and then get's nuked by three hybrid spider monstrosities, he's going to be a little unhappy.
One important lesson to learn as a DM is that only a fraction of what you envision actually makes it across to the players. It's like the story of the blind men and the elephant. The DM sees the elephant, but the players only feel the trunk and think it's a snake.
You have to learn to be blunt, and repeat and emphasize important things to make sure that what the players envision matches what's in your head.
I realise that you're only playing Devil's Advocate here, but by the sounds of this players actions I don't think it would have made any difference if the OP had described them as "humanoids", "a hideous purple creature that walks upright like a man with a face like that of a spider", or if he'd described them as "floating round balls with a large central eye and capped by a crown of eyes on tentacles". He was working under the false assumption that he was "safe" and therefore decided to do something extremely stupid and got himself killed. Now I don't know for certain but that's certainly the impression I'm getting: gung-ho and careless.