Complex sentence: He seeks out the man Kelly mentioned, and after killing him, meets with a friend of Kelly's, who informs the narrator that Kelly was divorced, was a drug addict, and had poisoned her son against his father.
Split into simpler sentences: He seeks out the man whom Kelly mentioned; after killing him, the author meets with a friend of Kelly, who informs him that Kelly was divorced with a son. She tells him that Kelly was a drug addict and that she poisoned her son against his father.