A lot of posts are telling folks that Marc Spector had Disassociative Identity Disorder (DID) before his near-death experience in the Temple of Konshu. This is wrong.
If Marc did have DID, he would have been ineligible for military service. Taking away his former Marine and Merc status is a huge part of his origin.
Nah there are comics where it shows him dealing with D.I.D. before going to konshus temple. There's one storyline where he gets dishonorable discharged and becomes a merc for lying about being mentally sound after he wanders into a minefield as one of his personalities and doesn't remember how he got there when he's found.