Lucifer no Migite

Three years ago, Katsumi Yu headed to Africa for a volunteer project as a doctor. However, soon after, a civil war erupted and he was captured by a guerilla group who was desperate for a doctor to treat the injured. He kept doing what he came to do, help people in need, and he does this for several years. But with the regime change in the country, the guerilla group was hunted down and Yu was forced into a situation of kill or be killed. He quits being a doctor soon thereafter and gets a tattoo of Lucifer on his right arm as a reminder of his past crimes.