Board Thread:The Flash discussions/@comment-807326-20181128021222/@comment-24152851-20181129092619

Oliver originally did it because he believed it to be his purpose, his father left him the List and he remained alive to fulfill it. Star City was extremely corrupt with police on the payroll of dirty corporate executives. The only justice to restore order was a vigilante operating outside the law. Sure Adrian convinced him he killed because he liked it, but that was not Oliver's initial intention.

Vigilantes are very different from serial killers. Vigilantes have a very specific purpose and believe what they are doing is right. Serial killers don't kill with reason other than satiating their desire to kill and they know fully well what they are doing is wrong. Furthermore, the modern usage of serial killer is almost exclusively for villains. So all in all, no Oliver is not a serial killer, he only killed when he gave a warning the person didn't listen.