I’m not sure what you mean by “believing in” comeuppance. It doesn’t automatically happen when people do bad things, it’s not a real material thing. People can do harmful things that sometimes cause people to react and punish them, and I’d say that fits your definition of comeuppance, but it’s not some guaranteed or spiritual concept. So I can’t say I believe in comeuppance, even when it happens.

If you mean in a sense of justice, I don’t really advocate punitive justice, as gratifying as it is. What comeuppance does someone truly atrocious on a mass scale deserve? There’s a point where you’d need to artificially prolong someone’s life for thousands of years of torture just to scrape the surface of the suffering they’ve caused to humanity (let alone other creatures), some proper “I Have No Mouth” sci-fi stuff would be the necessary fate to qualify as Hitler’s comeuppance. And what does it accomplish? Not much. In the end, just give them a bullet as quickly as possible to prevent them hurting more people, we can leave ironic fates to the novelists.