Well aliens aren't necessarily extinct, either...it is true that when cornered they become self-destructive organisms, but this is true of any creature. If you corner a wild animal it will fight back, regardless of its opponent or its own well being.
So far aliens seem self-destructive because they are forced to resort to extreme measures by outside influences. If left alone it is highly likely that the only time they would kill a member of their own brood is when that member would otherwise hinder the hive, such as those which have been severely wounded or deformed. Even then, the deceased's remains would be consumed or used by the rest of the colony.
"If anything in this life is certain, if history has taught us anything, it is that you can kill anyone." - Michael Corleone