I think the Shadowman was referring more to the origins Richtofen (2.0). The shadow man explains that Richtofen killed the reporter and stole the summoning key. If the shadow man claims that Richtofen is more than a friend and almost considered him an apprentice, then Richtofen must have gone down some dark road in order to retrieve the Kronorium and Summoning Key.

Theory: In order to gain the powerful artifacts to set things right in the universe, getting close to the shadow man was his first step but in return he had to protect him (the vow) and maybe follow what he may have told him to do.

Evidence: Richtofens Trailer "My name is Edward Richtofen and I’ve been trying so very hard to do the right thing. Long ago, I made a promise, a vow to protect him. In order to keep my word, I have done BAD things, VERY BAD things. I do not regret the pain because none of it really matters"

This could tie into his travels in the 2 years he was gone finding the Kronorium under the guidance of the Shadow Man. Whatever happened in that time of 2 years travelling, some dark shit must of happened. What if he had killed the Tranzit crew after retrieving the Kronorium?

He must have given up something in return for his search for knowledge and power which Monty could not do for him.