You can acknowledge that the Iranian regime is brutal and repressive and at the same time say that it is unconscionably evil to attack Iran with no reason or provocation or approval from Congress. The statements are not contradictory. Both can be true.
"How can you oppose war when Khamenei was evil?"
Simple. If you killed Donald Trump by nuking Washington DC, I would not mourn Trump but I'd still say you're an evil war criminal who belongs in the Hague. It's really not hard.