Reminder that Starfire comes from a warrior race where killing is acceptable and it is seen as an honor when defending oneself. After being enslaved, going through unspeakables, and being experimented on, do you think she’d be all bubbly and happy and not killing?
Even the origin episode in Teen Titans had her act more violent and uncaring. It isn’t until she forms a bond with the team that she become more concerning and caring about others’ wellbeing.