I didn't watch The Lion Guard, but I can guarantee you that their version of Scar's backstory, even if you don't consider it canon, will be better than whatever Disney will try to do for the "live action" movie.
https://youtube.com/watch?v=zoeOzR3EBfc
Summary taken straight from the wiki:
"In
The Lion Guard, Scar was born under the name "Askari" after his ancestor, the original Askari, and was at one point selected as leader of the Lion Guard, a team of lions who would protect the Pride Lands and the Circle of Life, as part of a tradition to all second-born children of the king. With this responsibility, Askari was given a power called the "Roar of the Elders". One day, Askari encountered a strange lion, who suggested that Askari should be the king of the Pride Lands. However, the strange lion led him to a trap where a cobra bit his left eye and left him his famous scar.
The strange lion offered to cure the cobra's venom in return for Askari's servitude afterwards, but Askari killed them both with the Roar in a fit of rage instead. After reporting the encounter to his brother, Mufasa, he was belittled and received the nickname "Scar" from his older brother and which would eventually become his only name. After that, Scar began to grow jealous and loathsome of Mufasa (as the cobra venom in his body began to corrupt him by bringing out his darker thoughts and tendencies) and came to believe that
he should be king of the Pride Lands instead of Mufasa. When the Lion Guard refused to help Scar usurp his brother, Scar infuriatingly used the Roar of the Elders to eliminate them. Due to using the Roar of the Elders for evil, the Great Lions of the Past took the Roar away and Scar lost the power completely and descended into depression, becoming shriveled and weak."
The venom apparently only removes moral restraints and Scar still had control over his actions. He wanted to overthrow his brother because Mufasa gave him a mean
nickname.
