Donovan is a the inheritor of a generational ability set that comes about once in a generation. During the ancient days, when monsters hunted humans openly and the night was long and full of dread, each of the archangels empowered a champion to fight back the darkness. Azrael created the saint of killers, someone for whom the taking of life is the most natural thing in their existence.
As time went on, the champions eventually drove back the night, and humanity began it's reign over the planet. The powers of each champion, however, was not tied to a specific bloodline, but was instead passed to a randomly selected individual upon the death of the previous holder. It then lurks just beneath the surface, waiting for a triggering situation to prevent itself.
For Donovan, that situation was the night that he and his sister were walking home from eating at their favorite restaurant when 4 men surrounded them, each armed with a pistol. One of them shoved the gun in his sister's face....and then next thing he remembers is his sister on the ground and then men dead around him.
His sister was unharmed, but she claims she doesn't remember much of the fight.
Since that night, Donovan's anxiety has been worse (dealing with the police can do that) and Donovan's sister has been trying extra hard to make sure that he is ok. All of this on top of the fact that he's noticed quite a few weird things going on with himself: He's in great shape, although he hasn't started exercising at all. He's stronger than he has ever been, and he is much more aware of his surroundings now.