Well, the first one, at least, the kill can't be denied. The pred was busy figuring out how to gut that guy, he really didn't see anything coming. And, well, the blades didn't melt as fast as any other alloy in any Alien movies do, so I guess they ARE resistant to the acid, but when you dig it right into the acid, it just can't resist for long. It did manage to cut a piece of the tail away and that made the alien much less dangerous. The net resisted to the acid quite long too. The alien... captain? did what he could, imho, and put up a good fight, he would have won if he didn't wait for the net to melt way.
And yeah, the pred removing his helmet was dumb, but he also seemed to be the n00b of the team.