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We know very little about Tyrese. We don't really know exactly why he did this, how he felt, what Fahim said to him, etc. I'm not defending him or saying Fahim got what was coming to him - far from it, but IMO Tyrese is not a diabolical serial killer. He was incredibly disorganised and delusional and I think a mental health defense is appropriate. I'd rather he be locked away in treatment for 30 years than in prison for 10 and then released because by then he will be in a gang and have that Rikers education to go on.
I think she may have been the motivator, unknowingly or not. Consider this - Tyrese has built up this persona of being a successful upscale tech ceo PA, he has made high profile friends and now he has a girlfriend. Fahim cut him off, forces him to repay the money, and now Tyrese's reputation and relationships are compromised. He has to continue keeping up appearances for this girl of being rich and successful. He rents the loft, he buys stuff for her - he couldn't keep doing that without access to the credit card.
Meanwhile, Tyrese has virtually no public social media presence, which is bizarre for someone in his position (I'd expect 5000 friends, more or the same instagram connections than Fahim, etc if he was really hustling) - that sounds like someone with bipolar, antisocial or schizophrenia to me - the American Psycho comparison doesn't work because Tyrese has no outward or reported behaviors of a charming sociopath.
Anyway the original point of my post is to watch how much you project onto victims and perps. Research the facts to help form your profile, don't play a movie in your mind.