Complex sentence: Despite his doubts, he assists Norman Osborn (Willem Dafoe) in his attempt to use the still-unstable serum on himself, but after acquiring his powers, Osborn's resulting' Goblin' personality kills Stromm (off-screen) in an uncontrollable rage.
Split into simpler sentences: Despite his doubts, he assists Norman Osborn (Willem Dafoe) in his attempt to use the still-unstable serum on himself. After acquiring his powers, Osborn's resulting' Goblin' personality kills Stromm (off-screen) in an uncontrollable rage.