McCall feels obligated to provide his services as the Equalizer because of his guilt over his days as a mysterious secret agent for the nebulous Company, a largely-undefined entity similar to the real-life Central Intelligence Agency. McCall apparently did nasty things for a good cause during his employ with the Company, and did not do certain things that now weigh on his conscience. His earlier acts of omission, and commission, drive him to act in an honorable, beneficial way as a sort of penance for his earlier life.
It is never too late to change. Regret may to a certain extent be unavoidable by those who reflect upon, examine in detail, and second-guess their past. However, a great man moves beyond what happened and focuses on the redemption of the present. Act now, and equalize your past. Be great now, no matter your past. Equalize, proceed and exceed in order to succeed.
From September 2010, http://raising-a-man.tumblr.com