Scar is an Ishbalen. Ishbalen people are men of faith, who worship a god named Ishvala. In addition Alchemy (or "Grand Arcanum" to them) is a taboo art that in their minds. To them It implys that Man can better upon Gods creation. Mattias, Scar's older brother, went against his own religion, and performed the most dangerous Alchemy known to man. Human Transmutation. This was in an attempt to resurrect his mother from the dead. But like all other attempts of this, it failed and created the Homunculus, Lust. This caused Mattias to go insane, and seek the Legendary Philosopher stone. Realizing that
he must have human life sacrifices to create this stone, he tattooed himself with the proper transmutation symbols in order to perform such alchemy. This would then let him become a living Philosopher stone himself. The lives he was gonna use were his own peoples, but before this could happen, his people were massacred by the State Military. While him and Scar were escaping, they were pursued by State Alchemist Zolf J. Kimblee. Who then scarred Scars face with his alchemy, and severing his right arm. Mattias was then able to retrieve Kimblees power enhancing Red stone, and fusing his own arm i n place of Scars before he died. Thus scar has a little bit of the Philosopher stone in his right arm.
From then on Scar was set to take revenge for his people, and his brother against The State Alchemists. He now is a serial killer who will fight anyone who gets in the way of his revenge, including Homunculus. His method of killing is using his right arms ability (which is partially Alchemy, which goes against his peoples teachings) by placing his hand upon a person forehead, and blowing their brains out from within.
Ability's and Alchemy
Scar is no Alchemist, in fact he knows very little about Alchemy or how to use it. Though with how his right arm is, he is able to do Alchemy without much experience or knowledge on the subject. With his right arm he does Alchemy a bit differently. Instead of transmuting an object he just decomposes it into an element. Its like taking the second step in alchemy, but not the third. His arm also gives him a subconscious knowledge of some bits of alchemy, like learning that the little girl Nina was a Chimera by just touching her(soon after killing her).
In the manga, instead of imploding someone brain, he would kill them by deconstructing their internal organs tearing them apart from within.
Scar is often at a disadvantage against State Alchemist, for he only has a close range attack at his disposal, so he has to be able to get close to his opponent, and at the same time is unable to defend himself due to his lack of Alchemy skill. Though he can decompose ground around his enemy, to hopefully get them to loose their footing give him openings to attack. He is also quite agile, and skilled in martial arts. With having these disadvantages, this proves how mighty of a Ishballen warrior he is, or was.