Sigma

Sigma, also referred to as Commander Sigma or The Commander, is the main antagonist of the Mega Man X series.Created by Dr. Cain, Sigma was considered the finest of the Reploids and was the first leader of the Maverick Hunters; peace-keeping Reploids who defend humans against their renegade counterparts. Although he was once honorable, Sigma unexpectedly goes Maverick in the beginning of Mega Man X and rebels against humankind. He defects to the Mavericks and assumes the role of their leader. Under Sigma's command, the Mavericks grow into a legion dedicated to the extermination of the human race.
In Mega Man X4, in Zero's Story, it is revealed that the event that made Sigma turn Maverick was after he first encounted and fought against Zero who is revealed to be Dr.Wily's Final Creation and was the patient zero of the "Sigma Virus" known then as the "Zero Virus". After a brutal fight where Zero gave him his facial scars, Sigma manage to barely take the upper hand and win the fight but it seems that this was the moment where he got first infected and later became the Leader of the Mavericks who keeps cheating death. The Day of Σ OVA, who takes place prior to the Reboot game Mega Man Maverick Hunter X, makes a slight change to that backstory since X is actually the one giving him his iconic facial scars during their first fight not Zero.