I think people hate the boiler because it serves no purpose in the hive (if it's even part of the hive, they seemed like they where outcasts to me). People "hate" the spitter because it has glowing green sacks on its head, which is a bit stupid no matter how you twist it. I liked everything about the spitter except that.
Well, the more I examine the aliens from colonial marines the more they seem like Termites (the spitter is like a nasute, and the warriors are like soldier termites, the queen like..well a queen etc), especially the boiler. When termites get old they turn white (like the boiler) and they go all kamikaze on attacking animals, blowing themselves up and releasing noxious acid (though not as dramatically as the boilers
). And even if Aliens don't age, it would make sense that if they were severely injured they'd do the same, and make use of themselves instead of just dying uselessly.