Not exactly warming to anyone

I'm about halfway through the anime now (they just completed the alien bird mission), and for the most part im finding myself more interested and sympathetic to the aliens? I mean is there any reason they need to even kill these things? The leek/onion alien mission was particularly disturbing to me, that poor leek boy! It wasn't like the kid was being hostile. Is Gantz just a dick playing around with these characters or something.
Also kind of pissed that the character I was warming to died in the latest mission. I guess I'll watch to the end, but feels like it'll be a struggle.