In either BG1 or BG2, if you're playing Evil in a bloodthirsty axe-maniac way, then Unholy Blight ought to be useful for slaughtering Flaming Fists, Amnish Soldiers, and sundry other guards who are fairly likely to be Good-aligned.
You might think that, but you'd be wrong. There are alignment bugs all over the place.
My favorite example is that in the paladin stronghold quest, I used Detect Evil to determine whether to trust a guy who said he was a fellow paladin. He was evil, so I killed him. Guessed wrong.
It wasn't one of those things where the story changes so that you're wrong no matter what you choose, either. He really was a good guy.
I know there were in the original BG1+2, that's why I was careful to say "ought to". I'm not sure to what extent Overhaul have fixed those bugs for BGee or intend to fix those bugs for BG2ee. If they clean up most of the nonsensical alignment errors (such as the example you mention, of a Paladin registering as Evil), then what I said should be valid, even though it was messed up in the original versions.
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My favorite example is that in the paladin stronghold quest, I used Detect Evil to determine whether to trust a guy who said he was a fellow paladin. He was evil, so I killed him. Guessed wrong.
It wasn't one of those things where the story changes so that you're wrong no matter what you choose, either. He really was a good guy.