Making a hidden power work

So, there's this high fantasy story I've got in the works, and without giving too much away, the protagonist has this incredible power which he can barely control and actually endangers his travelling companion. This power manifests as... well, think of something along the lines of Alucard from Hellsing; a raging mass of shadow and nightmare fuel which can rip apart entire armies. This power is justified in the backstory, but I want to play it for drama rather than as a deus ex machina.

How could this power be discovered, and what kind of problems would it introduce for the protagonist's personal life? I've already decided that it's going to cause a great deal of angst and guilt as his only friend - and the woman he loves - is already in severe danger just from travelling with him and he worries about what could happen if he loses control.

Any help would be appreciated.

Best way I can think of is to introduce a Secondary Character that the protagonist kills the first time the power comes out. Because he doesn't know about it, he can't control it, and his friend Bob dies doing something dumb and heroic (like maybe trying to snap him out of it).

Or he could kill his family with it when he was a child, after a petty argument that he took too seriously, if you want to play it for maximum angst.

Maybe he suddenly kills someone off. That's how it would usually go. But then make this person someone even the reader knows and is aware of. Make this person introduced much earlier than when s/he is killed, so when s/he dies accidentally, even the reader might be jolted.

Problems? Maybe alienating himself from others, self-loathing, being a Knight in Sour Armor, stuff like that. That's what I think of.

See, this isn't something that the protagonist is at all aware of despite his companion's suspicions. As far as he knows, the only abnormal thing about him is the fact that he's an orphan who got dumped on a doorstep.

I was thinking of having the superpowered evil side first emerge when his companion is in grave danger, and he almost kills her as a result.