Scar was, simply put, born evil. He was already pretty powerful as leader of the Lion Guard but his ego wouldn't let him be satisfied with just that and he felt entitled to have the throne as well, and season 3 of the TLG shows that cobra bite victims don't suffer any loss of control and simply lose moral restraints to negative impulses they already had in the first place. Kion got bitten by a cobra and yet never turned evil. And overall, I don't recall Scar having any genuine redeeming qualities.