Actually I don't blv so, I think the only thing to turn Natsu evil wil be if all his friends turn their back on him, Not even dispair itself can turn him evil, as long as he's got support frm his Nakama.
I think he'll turn bad if happy or someone in the guild dies, or maybe If makarov dies seeing as how they need to rescue him from someone or something.
NO....and when he'll be able to control END powers....we'll see normal Natsu personality.....in the end he'll use his dark powers for good purposes.............