Yeah ... thats because ... you konw they are movie MONSTERS. Why is it such a surprise if they act rather as monster then some typical enemy. After all we have no clue about how they think and reason anyway. The fact that they killed the marines very quickly well shows me that they have enough inteligence or that they cut the power, they cut the power man! How they cut the power, they are animals!. I mean we never seen how many of the creatures got really killed in the end. How they divide their force and resources. I dont think the movie shows enough to tell us here what level of inteligence they have, if they are capable of military tactics, if they are not better then ants, or if they are even capable of abstraction. I mean we should not forget that in Aliens it is the first time the creatures actually deal with an military force of professional soldiers, the first Humans they meet have been colonists of which probably only a handfull had even arms and any military traing, if at all. They had never to deal with something like Centry guns before and probably no heavy weaponry. But I do agree they take a lot of casualties and never seem to consider if something is worth the trouble or not. So yes, as far as "human" tactics and military decisions go, they are quite ... brute almost dumb. They are however rather animalistic in their behaviour. But this also make sthem somewhat dangerous. They dont have some kind of moral or feelings, not in the traditional sense. They can lose 5 or 500 and they will still try to achieve their goals no matter the cost. An enemy you can not demoralize. You dont reason with them or do any diplomacy, there is no capitulation if they get to you youre food, that's why Ripley said "Nuke em from orbit, the only way to be sure". Would have been a pretty boring movie that way though - and how Avatar should have actually ended. How do you know? Its not like you could see its eyes closed ... Also, as said, we do NOT know enough about its behaviour or inteligence to really tell how capable it is as far as the first movie goes. The only thing that is sure (and that is what the Android told) that it is extremly violent and hostile. And that it has probably no feelings or emotions. You know the original idea for the end in the script was that Ripley would be killed and the last scene would be the vesel releasing an distress signal as soon it got in range of other ships. That would be indeed quite an coinsidence. As said. We have NO clue what level of inteligence the xenomorph has. It definitely is not human like which has not to mean much anyway. It is just speculation, but I would guess it was there on purpose knowing the ship will blow up.