Death in the Family- A family member or loved one was murdered in a crime. You swore that no one would go through this again.
A Philanthropist of a Different Sort- You were born into privilege but even with all your money you couldn't stop the injustices in your city against those who were weak. So you took an unorthodox approach.
By order of God- You were commanded by God to enact justice in the streets. Every criminal put away is an offering to your God.
Because I'm a Madman- You are insane. Maybe you think you are a being from a different plane? Maybe you think that being bitten by a glowing rodent has given you the powers of a mouse? Maybe you charge thugs with nothing but your fists and some tights? Regardless, you'd have to be crazy to do this job.
Redemption- You used to be bad but now you're good, which is good for the city and bad for your former colleagues.
Friendly Neighborhood Hero- You're just sick of being mugged every day. Someone should clean this place up and the city guard won't do it.