I beleive it falls under a case by case basis. Fighting the thief that
killed your father would be a good reason to invoke Divine Wrath, while the
same character fighting just a run-of-the-mill thief would not. An elf that
runs into a goblin or pack of goblins while he is travelling down a road
would not invoke Divine Wrath. But if they were destroying the trees which
he has sworn to defend, then it may invoke the power. I don't believe that
it is a "Hulk" like power, activated anytime the character gets angry. I
tend to think it as more of the the type you see in movies, where the
character pushes himself to the last breath of life to kill the murderer of
his family. It only manifests itself in great times of need, sorta like an
enhanched adrinaline. It may not be as useful in this form, but it can make
for some great dramatic scenes. Hope that helps.