Marathon In-depth #30 Try Again - Marathon 1 - Bungie's Marathon Trilogy 

My guess is that Durandal doesn't leave himself on the Marathon because he has a cool road trip planned for himself on the Sfiera and doesn't see a purpose in leaving another Durandal on a colony ship that is parked around Tau Ceti and has no FTL drive. Besides that, maybe AIs are like us and are unnerved by the idea of having a second copy of themselves running around.

I always invisioned that Durandal knew where Leela was and was teasing that he was going to get her while virtually stomping around her core like some sort of serial killer, but never actually had any intention of harming her. Just for the lols, as they say.

Oh man, 20:20 I was like "Wait don't fire the!!........damn..." Yeah missile launchers and narrow corridors usually = bad news. ;)

Its a damn shame bungo would refuse to make an updated entry like this and insisted on cheater infested pvp bullshit. Destiny 2's shooting gameplay + a marathon tier story really would be waiting in the heavens. Since the clunkiness of the original games has not aged well compared to its contemporaries. Then again these are the political zealots behind "TRANS@BUNGIE" and insisting in regularly removing paid content from a live service game so you really shouldn't expect anything good from them.