The Wrong Enemy — Jane Evelyn Lebak

No one knows why Tabris, a guardian angel, killed the child he vowed to protect. The boy Sebastian got into Heaven, but the angels don't undestand why Tabris isn't in Hell. Instead God's given him a second chance. Another assignment. Another guardianship. Although he struggles to help this new child, a ten-year-old girl named Elizabeth, Tabris can't escape what he did with Sebastian. Elizabeth's co-guardian doesn't trust him at all, which makes sense because even Tabris doesn't trust himself. Everywhere he goes, the angels all know what he's done, and the only angel who seems to want him is a friend from long ago, now a demon. Shame and guilt follow Tabris like a shadow, but it's only the memory of the dead boy, and even though Sebastian still needs him, Tabris cannot face him. After what he's done, there's no way he can make it right. But his bright spirit is growing darker, and the other angels have realized that if Tabris can't accept the mercy he's been given, then he's going to fall forever.