Look, i don´t know why the first guy turned bad (though we can look at some of the people IRL that have lots of money and fame and get an idea... and also some of the classmates have a lot of "demon king" potential)... but i sure know why this one is going to, now we see that the only sensible choice as a ruler is the same girl that send him to die... so no revenge unless we want to sacrifice an entire kingdom
...Every new chapter is like a step by step demon king making manual, it looks like this girl had no choice but to summon a potentially awful monster in order to get more power herself, though why summon an entire class?, in her story only one hero was enough to unleash despair... and she goes and summons several of them?, yep... Fail from the start