The series is also this to the original comics. No "Boxing Glove Arrow" here (until a Shout-Out improvised use of a glove-on-an-arrow in S3's "Guilty")! To be specific, Oliver starts out as a cold-blooded killer in the series; generally the show seems to take influence from Mike Grell's run, which itself was notable for its Darker and Edgier and no-spandex or superpowers approach to the DCU.
Also applies to other superheroes featured in the series. Black Canary, and in particular Huntress, both of whom are killers; Huntress, indeed, isn't a superhero in this version, but an outright villain. At best, the protagonists in this series qualify as examples of Anti-Hero.
Ollie is MUCH darker than his comics counterpart, at least in the earlier seasons. For one, Comics!Ollie might be more than willing to hurt bad guys, but he doesn't break Thou Shalt Not Kill as blatantly as Arrow!Ollie does. Comics Ollie might have around maybe half a dozen kills in his entire existence as a character. By contrast, Arrow!Ollie might kill a dozen people an episode. Also, his idealism is less politically-motivated and more based on revenge for his father's murder. This is gradually downplayed as the series goes on, however, as Oliver's willingness to brutally torture and murder people has been called out many times by his friends and allies, resulting in him adopting a Thou Shall Not Kill stance in the second season (and his previous experiences as a killer being the source of endless Angst).
It's deconstructed in comparison to The Flash. When their characters cross over in "Brave and the Bold," they mention that they have the luxury of calling their bad guys metahumans. It helps them keep it from getting too real and serious, while Starling City is filled with much more real and darker threats.
Arrow's fifth midseason finale and The Flash's third midseason finale illustrate this. Despite both episodes' big climactic moments being pretty grim, they end with drastically different tones. The Flash episode ends with a celebratory Christmas sequence in which Joe finally hooks up with his coworker Cecille, Wally receives his Kid Flash costume, Julian befriends the team, the West house is visited by merry carolers, Caitlin uses her powers to make it snow, and Barry and Iris get their own apartment together. In the Arrow episode, on the other hand, everybody is miserable: Diggle is arrested, Curtis's marriage falls apart, Billy is dead, Felicity is distraught over Billy being dead, and Oliver is distraught over the fact that Prometheus duped him into killing Billy. The only consolation is Laurel's apparent return, which the next episode reveals to be a Yank the Dog's Chain moment anyway.
The fifth season is this to the rest of the series, featuring a Slasher Movie-influenced main antagonist and having Oliver return to killing after three seasons of abstaining from it (that last point does not go well). The 17th episode, "Kapiushon", is probably the darkest episode, not only in the season but also the entire series, as Oliver is physically and psychologically tortured by the villains after having been separated from his friends, leaving him an even broken man than before, which is saying something.
The eighth season drives home the bleak and apocalyptic nature of the Crisis. While the premiere is altogether a much lighter take on Season 1, with Oliver quickly learning to trust his allies (Diggle, Laurel and Earth-2 Adrian) and talking down Tommy from going through with the Earth-2 Undertaking, Earth-2 gets erased from existence in the closing moments of the episode by the Anti-Monitor, making everyone in that universe Deader Than Dead and all of Oliver's efforts to redeem Tommy ultimately pointless. Oliver also spends the entire season saddled with the knowledge that he dies in the Crisis, which considerably darkens his story arc.
Season 2 is this to Season 1. Barry starts out the season more jaded than ever. We have a Zoom who is a total scumbag, darkening every scene with him in it and antagonizing Barry. And the season ends with Barry losing his father.
It gets worse in Season 3 (the second half anyway) in which Barry accidentally runs into the future to see Iris murdered by Savitar and Team Flash failing at every turn to prevent it until HR sacrifices himself in her place via chameleon tech. It makes even worse when Savitar's true identity is a future version of Barry, created by the abuse of time remnants in an attempt to defeat Savitar, and wants to kill Iris to secure his own existence. The Stable Time Loop is eventually broken by HR's sacrifice and Iris fatally shooting him. To top it off, the season ends with Barry being forced to enter the Speed Force, separating him from Iris right before they are about to marry.
Season 3 is the culmination of this darker trend, as the next season goes for the Lighter and Softer route, featuring more camp and humor than even the first season, and ends on a much happier note than the previous three seasons combined.
Black Lightning: Darker than most of the other CW series, with its themes of racism, gang violence and corruption in the police. Tone-wise it's closest to Arrow, but even that series has more comic relief. Outside the Bloodier and Gorier streaming-only offerings, this is as dark as the DC universe gets on TV.
Crisis on Infinite Earths is the darkestArrowverse crossover by far. We see entire worlds burn, even some we know and love, which was actually much less of a thing in the original comic!
