This question is up for debate, and truly the only person who can answer the question as to why/how Taka received his scar is by the producers who made The Lion King themselves. There are many theories as to how he had gotten the scar, and most think Taka/Scar became evil because of him being "neglected". As for why he killed Mufasa, Scar thought of Mufasa as just another complication on his way to becoming king. He attempted to murder Simba as well, but during the movie that didn't happen. Basically, his evil lies in his past.

I agree with LionKing2001 on the fact that Taka was neglected as a cub but not on how he got the scar. I have been told he got it from a buffalo and it was Mufasa's fault he got it. This is a simple grudge that turns into much more in later life.
However, this is only what I've heard so I don't know whether it is right or not.

Because he was chosen to become king and not Taka/Scar.Ahadi did not trust him and
gave him a scar by hanging out with the hyena's.Then Scar wanted to become king
and the only way is to kill Mufasa and Simba.


Anymore questions just ask.