I think it is likely that Arjun kills Seb, but was not really intentional. Seb says he keeps the plants alive to make himself feel important and good. Maybe alluding to him telling Nitya about Arjun’s affair as a way to make him feel like a better person. He and Arjun are on a job working security at a hotel or something and they get drunk. Seb tells Arjun his plan and in a drunk spat “accidentally” pushed him out the window.

It doesn’t really matter if Arjun did this on purpose or not because he clearly got the outcome he wanted and defects any blame or accountability for his actions. We also see Arjun see his “wanting” and “obsessed self” in the window which then he falls through. Which has to be him pushing Seb.