Okay, Ash was an android, so technically he was just the product of cruel intentions. Still, the science officer was probably taking things a bit too far when he turned on his faithful crew for the sake of promoting the existence of an alien species. Moreover, Ash's betrayal is two-fold: firstly he breaks quarantine procedures by allowing an infected crew member back on board, and secondly, he attempts to kill Ripley by stuffing a rolled up magazine into her mouth.
That initial act of treachery led to the deaths of everybody aboard the Nostromo, bar one woman and a cat. What makes Ash's behaviour more shocking is that he was so likeable beforehand. While he might have been a little wrapped up in his work and his adoration of a creature that just tore through a man's abdomen was certainly questionable, he was polite, rational and occasionally compassionate.
It's only on a second viewing that we begin to see Ash for what he really is: a cold, calculating machine who is merely carrying out company orders. Though it's not as if that revelation made a blind bit of difference to the crew of the Nostromo.