Sometimes I forget Lucy and Natsu are just animated characters who doesn’t exist and then I get sad.
My mom’s opinion:
'If Natsu turns evil, no one will be able to kill him. Not Gray, Erza, nor Makarov or even Guildarts. You want to know why? Because the only thing that can kill a demon is an angel, wich in Natsu’s world (I think) would be a celestial spirit mage. Which means only Lucy can defeat Natsu. After all, she is his only weakness’