Talk with Mabuta in the strip club and make a deal with him before your whores work more than one day
Your slaves and whores also need protection. Mabuta is the best choice for starting out and he will also agree to provide protection with his Nigerian gang if you talk to him a second time
How to kill Mabuta
Hitmen are hired from your gang members. They have to own all the firearms, complete firearms training, be a high level soldier (greatly preferred), and have some kills from being the 'Gunman for robberies'. Then the option will appear to make them a hitman. You can then ask them to eliminate Mabuta
Always have your highest level soldier be the 'Gunman for robberies' and then once he has a bunch of kills, assign him as a Hitman to kill Mabuta
Your gang members must own all the firearms in order to begin firearms training. Some of the firearms are expensive so if you've just started the game, just give all the firearms to your 'Gunman for robberies' only and you can give them to the rest of your gang once you start earning lots of money