Scar was one of the first Xenomorphs put into captivity by the Weylan Yutani Corp. He was classified as (Xenomorph 1212)and raised by the scientists, which taught him to become very smart. After about two weeks though he tried to escape and failed, showing the scientists he was going to be very dangerous when grown.
After the power out in the research lab, Scar escaped and with the Queen dead, took charge of the remaining Xenomorphs. He lead his pack into a cavern and there they went into hibernation for the next couple of weeks.
Later marines came to investigate, but were ambushed by the creatures. So they started to retreat to their craft where Scar and the Xenomorphs that were still alive had hid. Once on board they killed the humans and went into another hibernation.
After their second hibernation, the ship had crashed on another planet owned by Weyland. Once there, they attacked the lab and freed the other Xenomorphs. The queen of these Xenomorphs was still alive and replaced Scar as leader.
Scar started becoming manipulative and had a urge for power, but did not betray the Queen. He did become second in command of the hive he now lived in and stayed there for a long time.
Many years later more marines came, but were prepared and bombed the planet before landing. The bomb killed most of the Xenomorphs besides Scar who was captured and brought to a lab on another planet.
Scar than during a minor power outage escaped again, leaving behind the other Xenomorphs and searched the rather large planet for a new hive to join, but on his way through the jungle he ran into a predator. He fought the predator and killed him, but was left with a scar on the right side of his face, giving him the name Scar.