Although he has the reality stone he already lives in his own reality and everyone is just part of this.
He feels like the only way to save everyone to by removing half of eveyone so that civilizations can continue to grow.
Consider Ra's al Ghul's goal of completely killing everyone in Gotham just so he can rebuild it. He even says something like, for civilization to continue it needs to fall and restart.
Although both characters are evil thaons believes that the only way for life on every planet to continue is for them to lose 50% of their populations, which would fix a lot of issues like, overcrowded, global resources demand gets reeuded and allowed for more time for the races to come up with other ways to progress, Etc.
Is he evil in his mind no. Is he evil to us looking at him, yes HELL yes. But again to him he's doing what no one wants to do for life on every planet to continue. In his mind he's the savior not the villain.
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