For his self-interest. Yes. Bada's cuzzo isn't a good person at heart and lies and schemes. Doha is too intelligent to let people he knows are going to get in his way stay alive. So for the sake of his self-interest it makes sense to kill off annoyances before they're actually annoyances and he can simply blame the murder on the serial killer to make things smoother.
Impressive in my opinion though~~
Wdym his cousin is bad?? Why
Doha kinda went on his own investigation to find out more about Bada's cousin, revealing that Bada's cousin doesn't actually care about or talk to Bada on a close level and he exposed that he's a liar and a plotter as well (this was a couple chapters back) but this can also be proven by Bada's cousin's dialogue in this recent chapter. He snoops into Doha's bags and basically plots to expose the 2 of them infront of everyone for no apparent reason other than just for the fuck of it. But Doha kills him before he can do anything.
[this is all just observation and speculation by the way it isn't completely accurate info just what I believe to be the case~~]
did he have to do that