Well if he lets his jealousy control him... Maybe. But in season 5 they built up an older brother/younger brother relationship between him and Lloyd, like trying to redeem him from almost killing Lloyd in the previous season.

Okay, I seriusly agree with the idea that Kai could become evil. He actually almost did. In season 4 he legit leaves his sister to get eaten by A giant snake. In season 7 he attacked his parents. And you can't deny that he is super Jelous about Lloyd

He even says in episode 35 that he should have been the one to save everyone from the overlord, and that ge should have been the green ninja. Immagine this: What would happen if Nya died. What would Kai do? He would become enraged, and would unleash his anger on everyone. He would turn into a villan.
So, is ir really too far to immagine Kai becoming evil? I think that in season 4 we saw his true colors when he was holding the staff of Elements.