Well if I was a primarch chances are I'd be someone a little rough such as good ol' Russ. He seems like a guy I'd like so I'd probably follow him. If I'd have been there on Prospero maybe the Thousand Sons woulda ate some extra bolter shells and the 13th would still be here kicking it with the other
SW. Although knowing my luck as a primarch I'd probably would've ended up on some world not even close to being noble savages and resembled a hive world (despise being surrounded by people >.< ) or stuck on a world that is more like this world when it comes to humanity's integrity... in which case (no offense to humanity... but we suck...) I'd have probably worshipped Khorne and charged head first into battle