Wow, props to you for volunteering at a kill-shelter! I would certainly have a difficult time working at one, but it's necessary and difficult work! It really is quite upsetting to see how cruelly animals have been treated, especially the ones who can't recover.
We had a coonhound this last winter with a hard lump on her head that the veterinarian we work with deemed was caused by a hard hit to the head. We ended up discovering after behavioral issues (sudden aggression, excessive barking, anxiousness) that she had been hit so hard that a lobe of her brain had detached and that there was nothing we could do. I cried so hard when I learned she was euthanized. (I had worked a shift with her the same day she went for xrays and subsequent euthanasia. I wish I would have known, I would have stayed with her and pampered her) She was wild and sweet and obviously couldn't help her neurological problems.
It's hard to see abused animals, and some outcomes are difficult to swallow, but it is the most rewarding feeling seeing an emaciated dog or cat gain weight, or learn to trust humans again. I'm so glad there are people like you and those I work with that care just as much. Makes my heart happy, really.