
I feel strongly that K committed the murder, and fled the scene while Gusam was under the influence of something from the cigarette he shared prior to it.
Then on top of that, K made a comment to the black haired guy at the police station that insinuated he would do something bad to him if any information were divulged. I don't think that black haired guy witnessed the murder, but definitely has information on K's activities that would put him as a top suspect.
It's ironic that Gusam actually didn't commit a murder (in my opinion) and Lee actually ended up committing a murder (and wouldn't have if he never met Gusam to be put in that situation). Their agreed suicide was very beautiful and tragic, because they were too far gone to step back.
Y’all leave my dude alone He is dense but the dick game is on point