Warning: spoilers ahead for Episode 17 of Arrow Season 5, "Kapiushon."
Season 5 of Arrow has primarily been about bringing Oliver Queen's journey full circle now that his five years of flashbacks are almost up, and the flashbacks have mostly been filling in a lot of blanks. We've seen Oliver becoming a part of the Bratva, learning Russian, and gaining many of those skills he had as the Hood back in Season 1. In "Kapiushon," he finally even got that Bratva chest tattoo we've been looking at on his chest for the past five seasons. "Kapiushon" did more than just fill in blanks, however; it also made a huge change to Oliver's backstory by revealing that his real reason for killing all the time was that he liked it, not to honor his father.
"Kapiushon" was the Season 5 mostly-flashback episode, so the majority of the hour was spent following Oliver's action in Russia as he went so crazy on Bratva prisoners that even Anatoly was appalled. Oliver was killing when it was not entirely necessary, and he was so far gone that it didn't even register to him that he was really going too far when the seasoned, murderous Bratva leader thought he was crossing too many lines. After Oliver -- looking positively high on the thrill of his murder spree -- decided to kill (or attempt to kill) Kovar, Anatoly realized that Oliver was only killing because he enjoyed it.
In true Oliver Queen style, our Emerald Archer immediately denied Anatoly's accusation, but torture at the hands of Prometheus in the present forced Oliver to face his own demons and admit the terrible truth. Pushed to the edge by the physical torture, emotional torment, and traumatizing trick that made him believe that Prometheus has killed Evelyn, Oliver bellowed his confession:
There's really no ambiguity. Oliver was much too far gone to possibly pull off a lie, and there's no way Oliver had his wits about him enough to fool Prometheus. The truth is out: back in Season 1, he was picking names off of his dad's list because he enjoyed the hunt and the kill, not because he was a staunch believer in justice. He killed because it was the more satisfying option for cleaning the streets of Starling City, not because he thought it was what his father wanted. The revelation marks a huge change to the backstory that Arrow has been building off of for the past five years, and it makes it hard to predict what the show will deliver next.
All of this said, Oliver isn't a monster for the reality that he likes killing. Obviously, he's not the most emotionally balanced guy ever to wield a bow and arrow. Still, he's gone to great lengths over the years to avoid killing. Sure, he made exceptions, but prior to the beginning of Season 5 when he was back on the killing bandwagon, he fought his instincts to take lives.
As far as I'm concerned, he deserves more credit for his struggles to not kill in Seasons 2-4 than he did before the reveal. It does paint his Season 5 killings in a pretty terrible light that doesn't 100% make sense to me as the show is supposed to be bringing him full circle, but I don't believe his character is ruined for it. We can only wait and see what happens next.