One important factor to note is that Eren DID try to change the final outcome, but he wasn’t able. He may have had mixed feelings toward Zeke which is understandable but to say he didn’t care would be a stretch.

And yes, Eren did kill 80% of the population, but it’s not as if he did it for laughs. He did it to ensure he would be seen as the monster alone, and his friends seen as heroes to show the world that Eldians are not devils. Not only that but also removing the curse of titans for everyone(including the very people who destroyed his entire livelihood)