The idea I think, is that every soul ultimately goes to either Satan or God. And that it's our choices and character who determines whether it's Satan or God that gets it. Yes, God gave us our souls -- in order for us to freely determine whether we'll be giving them back to Him.
Mist wanted power, admiration, etc. His choices and character gave his soul over to Satan. Rikk wanted the safety of both his friends and the world. Despite thinking he was making a deal with Satan, this gave his soul over to God.