Mephisto probably just wanted to see what Norman was willing to trade in for the power and wealth he sought in life, and getting Harry's soul was the somewhat inevitable byproduct of that when factoring in how ultimately selfish and egocentric Norman was. As for Mephisto becoming the ultimate adversary of the Avengers, going by Jason Aaron's run with the Avengers 1,000,000 BC, the Avengers have in one form or another over the course of MU history risen up to oppose Mephisto's machinations to gain total dominion over Earth, so Mephisto, being unable to directly take on the Avengers for some reason, manipulated various people and factions across that same history into becoming his pawns and agents to counter the Avengers. In that same vein, it's been theorized by some that Norman rising to power the way he did in Dark Reign and using it to undermine the Avengers might well have been another salvo in Mephisto's secret war on the Avengers, and somewhat similarly, maybe the whole point of OMD is to corrupt Spider-Man, with his own unique position within the superhero community, into Mephisto's greatest pawn against the Avengers.