One aspect I like about the series is its overarching dark tone. Also, it's more akin to real life in that anybody, right or wrong, unforgivably, can die. This is something I feel is lost on people when Alien 3 comes around; that this series for a long time before has been heartless and killed off many innocent and sometimes central characters. I will admit that Alien 3 accentuates it more; Hicks and Newt's death is jarring, but that's what life is like, and it is still very much in line with the ruthless nature of the prior films.
However, when you think about it, ALIENS is actually the darkest of them all, when you consider the amount of families that died at Hadley's Hope. I mean, it would have been a massacre, and no other film in the series has come close to depicting that brutality.