And here comes the whining about "Y didn't MC just kill him?"
Why didn't MC kill that guy? He was threateening MC and offered him the options: 1. Get ratted out by me (eventhough you saved me before, or at least helped me to not reveal my pay2win-items), which would put you into mortal danger. 2. Become my slave (and by seeing my behavior you know, what a scumbag of a master I would be.) Or 3. Make my shut up about that matter (, but leave me as a lose end and leave the possibity open, that that information gets extracted form me in another way. (Torture, blackmailing, contract-canceling, mindreading, etc.))
OR MC's own no.4: Just kill him and take his stuff, leaving no loose ends. Well, except for being wanted for murder, but then again no one knows his identity.
And for a more moral pov: This guy is a mortal danger to him and his freedom. There is also no government, that can protect him, even if he tries to take a lawful approach (something that works more or less in our world, but not in that fantasy). He also has to protect his sister. The contract IS a good reason for not needing to kill that lowlife, as it would ensure some safety from information getting spilled. But then again, those conditions are not good enough. The other side should also have offered becoming a slave on his side. Also it is to be expected, that the other side tries to cheat. And when it happened, MC was troubled for a moment. So there was a real danger, that MC could have become overpowered, if things would have been a little different. (MC didn't know about it beforehand, but should have been at least aware of the realistic possibility.) Overall MC shouldn't have endangered himself and his mission to create a save haven for his sister. There have been worse excuses to kill a person in other mangas, than the MC has in this case. His and his sisters lifes are in mortal danger, and there is neither a way to avoid this situation nor a lawful way to confront it. He can't run, there is no help: That's it.
Connections are very important and having that guildmaster as a slave might help a lot in the beginning, but that also wasn't an the table. Simply not worth the danger.