Because ultraleft believe that revisionism is worse than capitalism, do they ally with capitalist to destroy revisionist?

Depends on what ultras we're talking about but broadly speaking in the West they will tend not to engage in politics in the real world. I wouldn't call it allyship with the capitalists, it's more like complicity.

Your question is a bit strange. Revisionism is indeed bad. Let's take Kautsky. Kautsky was not worse than capitalism later, he was a part of it. But where did you get it from that "ultra-leftists" think like that? There are just so many ultra-left groups and currents historically.

For example, there are anarchists who think they can "escape" from capitalism by owning a piece of land, practicing self-sufficiency and founding a commune. Of course, this is all nonsense. But it's not as if they stand up and are allies of capitalism and imperialism.

There are also anarchists who think Russia is so bad that it is the lesser evil for them to cooperate with NATO.

Anarchists are ultra-leftists. But there are an incredible number of differences. There are also ultra-leftists who are not anarchists of course.

Speaking of anarchist. They were helpful in the Ukraine during the civil war and allied together with the red guard. Afterwards they were suppressed, but helped again and again against the Ukrainian nationalist and the whites. They were ultraleft but not allies of capitalism.

Ultraleft is something very broad. You can maybe say, that "in the end" they are helping capitalism and this applies. But that they all are that way how you describe it, I deny it. Some of course. But I can't remember for example that Bakunin as example were this way.

I am talking about anarchism because "ultaleft" absolutely applies to them completely.

Edit: Made something bold

Perhaps not Maoists. They usually follow the two line struggle, where they debate with revisionists to eventually prove that they aren't following a proletarian line, but a bourgeois one. For them, MLs are mostly misguided by revisionist party leaderships.

Things are a bit different in wars, where Maoists will fight revisionists in the wars they consider unjust (like in Afghanistan, where the Maoist party opposed the Soviet intervention with arms). The same applies to wars of aggression led by imperialists, like in Afghanistan 2001 and Palestine.

In those cases, they will ally temporarily with national libertation movements, even if their ideology is reactionary. That's why they defend the palestinian and afghani resistances so much, even if some sectors of those groups express reactionary ideology.