Manny is the drug dealer murderer. He is doing that because Issac said it was god plan for him to kill those that bring drugs into the city. He was able to manipulate Manny in this way because Manny’s brother died by the hands of one of those drug dealers.
The first time Manny killed it seemed like it was an accident/spur of the moments and ran to the church for forgiveness but Issac used his mental instability to his advantage to where they formed this deal between each other.
From what we see, Manny seems to go blank when he’s ordered to murder someone (like his friend with little siblings that he killed)
Issac is also very obsessed with Manny and is using Manny’s instability to sleep with him now as well.

Someone needs to explain to me what guilt Manny is carrying that's making him constantly give in to this rapist piece of shit(╯°Д °)╯╧╧ I dont get it! He's infuriating me constantly hiding things from the seme not saying the seme is any better but ugh Manny is pissing me off! Hes so brainwashed by this fucking guy that I cant even be happy about the him and the seme finding happiness together! ( ̄へ ̄)im seriously getting to the point where hes about to be added to my most hated uses ever becuz hes just not doing it for me at all. ヽ(`Д´)ノ