I will be posting Blaire's journal entries here, so that should answer some questions. But I'll try and answer this one the best I can. They do scare off angels. They're massive. Blaire once wrote that they could pluck the wings from an angel like a human plucking them from a fly. Suicidal souls are the only ones they care about. It's not KNOWN, but it's believed that they consume the souls, which is how they've garnered so much power. They do have some weaknesses though. The giants are hard of sight, and their reaction time is slow. Killing one is out of the question. But stunning or injuring one is not. According to Blaire; suicidal souls have reached Heaven by overwhelming the giants with massive quantities of angels and dispatching a few to use the power of angelic weapons in order to slow the giants down. They might be slow, but due to their size, one step gets them quite far. If the suicidal dead is important enough to Heaven, then high-ranking angels (who hold more power) might show up. The quickest of the angels will fly low (the giants have trouble seeing anything too low to the ground) and bolt for the soul as its kin attempt to hold the giants back. This kind of effort is rare, though. They'll definitely do it for a divine soul who committed suicide. Or a child. Special cases. An average person who kills themselves is getting taken by the giants. Blaire followed many into the Dome. She was on a quest to learn what each kind of soul was in for in the Dome. She followed suicides, children, murderers, average joes, babies, witches, etc.