I'm not. I don't think any of his kills were justified, besides maybe the 3 guys in the subway. Even though we don't know if that's even actually how it went down. And even if it did go down like that, hunting down and killing the last dude was way fucked up.
It just got worse from there. Killing his mother was extremely fucked up, no matter their past (which we don't know is real or not). I took it that it was implied that he killed his "girlfriend" after entering her apartment and possibly even her child, considering that it cuts to him back at his apartment laughing hysterically shirtless with ambulance lights in the background, and we never see her again, which is ULTRA fucked if if that's the case.
Him killing his work buddy that "fucked him over", which we're not even sure happened like that at all. The whole interaction with the guy giving him the gun was super weird, and reminiscent of his interactions with his "girlfriend". Him and Gary seem to really want to be there for Arthur when they come over, and Gary seems 150% caught off guard by the murder. Of course it's murder....that will catch anyone off guard, but his reactions to me made it seem like the dude had done nothing whatsoever to potentially deserve that. "Why would you do that Arthur?!?"
Then of course the Murray stuff which is just completely inexcusable even if you're going purely off of the story being presented. So no, none of these kills are justified at all to me. If you're going off what the movie's presenting, there may be some that are easier to let slide than others, but that in my opinion is kind of the point of the movie. Arthur is justifying this stuff in his own way. If we go off of his narration, there's no telling what seeds were planted throughout this movie and his mind for him to be able justify going down this path. When he's still trying to fit in somewhat, but his mind is still completely frayed. Not to mention less than halfway through the movie he comes off of all his medication. It's not until he's fully Joker, when he's on the Murray show at the end, that completely clear what his motivations are. Because that's when his mind is the clearest.
I think even with Arthur's more sympathetic towards himself (or whatever character he's actually talking about) narration, he still can't help but slip in some shit that hints at his true nature. Hence hunting down the subway guy, the whole interaction with Gary after murdering the other co-worker, ect. He's the Joker, and he can't help it.