Its believable emo though. Not those gundam seed kind.

You know, it is hard to be not emo when you are being abandoned, have no prospect of resupply, have to make your own rules, trapped in hostile environment like space, a ruthless enemy on your tail and people trying to take your stuff/dignity/humanity/sanity because they can. Plus your best friend can be your worst enemy tomorrow out of desperation and the ship have some weird  juju going onboard. Civilization is just a veneer that can be stripped away fast in blink of an eye. This anime shows why and how brilliantly. 

As someone said, they have a reason to be emo, not carte blanche Seed emo shit.

well "believable emo" level is subjective to every individual XD