I’m like 95% sure Adrian Chase is Vigilante, and I also think there is a possibility he could be Prometheus, but why is he doing it? There are many traits that Oliver and Adrian have in common including both mentioning to have been through some kind of hell. Some things Adrian says are very similar to how Oliver says things. Adrian mentions in 5x03 that he met Oliver 12 years ago when he picked up his girlfriend but he claims “its ok I don’t hold grudges”. The way he said this very Oliver-ish when he covers something up by directly telling the truth a prime example being 2x02 when Thea asks how he got in the club and he replies “when I ran the club I installed a secret exit”. Another example is in 5x04 in the flashbacks Oliver says to Kovar’s man in prison when he tells Oliver he’s not intimidated by him, “he’s not in here, I am”. In the present day of that same episode Adrian says to Church’s man in custody when he tells Adrian he’s not intimidated by him, “He’s not here, I am”.
One has to ask since Adrian seems to be a foil to Oliver what tragedy did he endure to send him down the path to become Vigilante. I think it’s related to that girlfriend remark he made. Was it possible his girlfriend, the one who Oliver “picked up” was actually Laurel Lance? Adrian said Oliver picked up his girlfriend 12 years ago which at the time would mean 2004, and we’ve seen that Oliver and Laurel had been dating for sometime in 2005. Adrian is a D.A. just like Laurel would have been if she wasn’t killed which means they obviously both went to law school and Adrian looks around the same age as Oliver, who was born in 1985 like Laurel was which means Laurel and Adrian could have attended law school together. Perhaps Adrian and Laurel could have been work associates even after breaking up and Adrian could have pretended to be fine with it, but really did still love her.
Then with Laurel's death last year at Damien Darhk’s hands Adrian could have been pushed off the deep end in grief which Oliver in a way also has been, as he is now killing again because he hesitated against Darhk multiple times and by the time he did the love of his life was dead and he will probably always blame himself for her death. Adrian could have been affected the same way but doesn’t mind causing collateral damage as he probably is a little deranged from the loss. Also Vigilante seems to be trying to push Green Arrow to do what is needed to stop the war on crime, by pushing him to the most extreme legths. This season is about legacy mainly Oliver trying maintain his and honor Laurel’s but as Prometheus is living proof of, Oliver’s legacy has good and bad sides but so does Laurel’s. Maybe Vigilante could be one of the bad sides because her death prompted Adrian to be Vigilante.
Just a thought but we know virtually nothing about Adrian Chase and his reasons for being Vigilante, as I assume he is, but they usually hint without actually hinting towards revelations regarding character identies. I always thought Adrian's comment about Oliver hocking up with his girlfriend was suspicous, especially given the timing of Adrian and his girlfriend dating in 2004 and Oliver and Laurel dating around 2005. I think it would be a really sad revelation but also fitting if more than just the inner circle of Team Arrow were affected by Laurel's death.