Because in "my opinion" of all the characters in the TLK universe, Scar has had it pretty bad during the course of his childhood. living in the shadow of his brother Mufasa, only being shown affection by his mother and Sarafina, almost killed by waterbuffalo or his father Ahadi, give or take. and finally being denied the his equal share and/or chance of being king.
and when you look at it more closely, all Scar really wanted was love, affection, attention, and to feel important just like his brother. so if you ask me is Scar truly evil? No, i honestly believe that deep down Scar never wanted to be evil to begin with. but thanks to the certain events that took place during his life Scar was morphed into what we see him as in The Lion King. but i truly believe that none of the stuff that occurred in TLK 1 would have never happened, if Scar was given the same love and attention that they gave his brother, and that is why i love Scar so much, besides the obvious reasons
TH.