I hate when games won't let you kill certain bad guys yourself.

"Oh, you want to kill this baddie whose been terrorizing you throuought the whole game and you hate them? Too bad, they will die in a cutscene."

@Chameloshi: What about bad guys who you want to kill, but can't because you're playing a prequel and they're alive in the original? Bob Page, Revolver Ocelot, Giacomo, Geldoblame, Skeed, Valee, etc.