Because he's jealous and knows he'll never become king once Mufasa steps down and Simba becomes king. So, Scar had a plan to get rid of both of them, sa pamamagitan ng killing Mufasa and make Simba run away.
In a fight, yeah, but Scar was smarter. Anyway, from what I've read/heard, he was treated really badly in his childhood sa pamamagitan ng his father and Mufasa. But there had to be a villain for a movie and Scar is just the one.