For his interactions with 
, we know two things. Her father hated 
, and maybe asked her to kill him one day, and is considered as the primary suspect of his death for most, but he denies it.
So, either he is lying, or he didn't do it. What do you think?
What if swain and kat's father encountered a demon and they both tried to kill it but kata's father ended up dying while Swain finished the job but somehow the demon's hand is now swain's left arm?!
Marcus is still missing, so we aren't entirely sure.
With Swain being as smart as he is, it's not likely he'd leave loose ends around, unless there was a very good reason. It's quite likely Marcus is dead, but there's an interesting line in Sion's lore where he mentions someone telling him to "beware Ravens", in the depths of his grave under the monument.
Personal theory, that was Marcus, and Swain is keeping him the same place he stores Sion. Alive? Maybe. Why?
My guess would be "demon sacrifice", but again, personal theory stuff.
