The others may be valid criticism, but from a storytelling perspective, he most certainly did not kill Wash for no reason.

Wash was killed at that moment so that first-time watchers of the film would suddenly believe that every single one of the characters might be about to die -- because fucking Wash just died. Wash was killed so that when Zoe, Kaylee, Simon, and even Mal and River went down with injuries or flew into the breach, you believed they might really die -- because fucking Wash just died.

It was done for that effect, and a less popular character wouldn't have cut it. From my own perspective, it was successful. I could no longer be certain that some of my favorite characters weren't going to die.

(Fun fact: before Serenity came out, sometimes some friends and I would discuss ideas about where the Firefly tv story might have gone if it hadn't been cancelled. My idea for a season finale, probably in the second season, had Wash dying too. After learning that Zoe was pregnant. Before he died, he'd ask Mal to take care of them.)

(Geez, maybe I'm just a monster.)