You know, I'd argue that Diabolico didn't neccessarly turn good. Sure he helped to take Bansheera down in the end. But that was more to get back at her for everything she did to Loki and himself. I do think he had a genuine friendship with Loki. But if Diabolico survived the war with Bansheera, its very possible that he would have picked up where she left off. Only for his own personal gain this time.
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