It was a cost cutting measure. Somewhere in the myriad of special features on the Quadrilogy Blu-ray, Michael Biehn mentions, "I was paid more for them using my picture in Alien 3 than I was for my performance in Aliens!"

Ticking off and "alienating" your audience (heh-heh) might be considered "brave" - but it also could be called "stupid" because the majority of folks who went to see Alien 3 were looking forward to finding out what happened to them! What they got was, "Oops, we killed them off - while you weren't looking." The movie bombed and paid the price for it.

I think to be "brave" about killing them, they would have had to approach it like Psycho. Wipe them out midway through the movie when it means something to audience. As it was, I see it as a cost cutting cop out. So did most of the fan base.