Each member of the Suicide Squad mirrors the Justice League in some way:
Harley mirrors Batman with her reliance on gadgets and weapons as well as her intellect. It can also be argued that she mirrors Wonder Woman with how she’s the only female member of the team.
Deadshot mirrors Green Lantern in his role as his team’s sniper. John Stewart was also a sniper before he received his ring, and he relies heavily on gun-based constructs during his boss fight.
Captain Boomerang mirrors The Flash not only with his use of the Speed Force, but also with his penchant for witty remarks.
King Shark mirrors Superman by being the strongest member of his team and his status as a demigod.
Hell you could arguably say Amanda Waller picked them specifically for this as almost each member has a previous history with their respective leaguer as well as experience with how they fight and backed up by A.R.G.U.S. Tech and support from Waller stand a better chance of actually succeeding compared to when they fought each of them on their own.
It also helps explain why the Squad goes after one leaguer at a time instead of trying to take them all in at the same time. Along with having the power in numbers where one leaguer one ups their counterpart the other three can fill in that niche whether it be raw power, intellect, or skill.
Why does Brainiac become a copy of the Flash in the final battle against him? Because with his intelligence, he can think fast.
Fridge Horror
It’s probably for the best that the Squad actually did succeed in killing the mind controlled members of the Justice League because it’s very likely that the Leaguers especially Batman would not be able to live with themselves after finding out what they did (which encompassed killing millions of innocent people)
So with all of the members of the Justice League dead, it's safe to assume that this world is screwed if someone like Darkseid attempts to attack.
Fridge Logic
With the confirmation that the Green Lantern Corps exist in this universe, it becomes a bit hard to believe that the Corps would let Brainiac, a being who is dead set on large scale conquest, take control of one of their members and his Power Ring and proceed to do nothing about it.
With Batman brainwashed, he should have already been aware of the contingency plans he had put in place to neutralize the Justice League in case they went rogue, with many of the anti-League weapons he prepared sitting in his own bank vault. Yet despite massacring the bank's security detail, the brainwashed Batman allows both Wonder Woman and the Squad to access it seemingly without a fight.
Perhaps this can be explained by some variant of the Zur-En-Arrh protocol of Batman RIP is in effect, where Bruce blocks various parts of his psyche in case of mind control.
Similarly, The Flash is sent by Braniac to capture Luthor and extract/torture information out of him. It somehow doesn’t occur to Braniac to just have Flash zip Luthor up to his ship and brainwash him as well, which gives the Suicide Squad an opportunity to save him. And in the end The Flash simply kills Luthor and moves on without getting anything useful out of him.
The game's decision to Retcon the previous Deadshot as being an imposter, as opposed to simply retconning him to have always been black, makes exceedingly little sense the more you think about it. Not only would this mean Batmanwould've had to have fallen for the fake "Floyd Lawton", even though he doesn't even have the same skin color as the original, but also means the imposter and the real Deadshot both have a daughter named Zoe and the same set of Improbable Aiming Skills. In fact, because the player is in-control of Deadshot's aim this time, it ends up coming across like the "imposter" is actually a better sniper than the real Lawton. The in-game audio logs revealing the original Arkham Deadshot is from Earth-2 doesn't completely fix the problem, as it still raises the question of how Earth-2 Deadshot has been able to run around in a foreign universe for over a decade without having any issues himself or anyone else noticing.
This game states that the Justice League wasn't formed until after the events of Arkham Knight. Fair enough, but on the previous games we had easter eggs referencing Superman, The Flash, Green Lantern (Both Jordan and Stewart), Firestorm, Martian Manhunter, Hawkman and Hawkgirl, Black Canary, Green Arrow, Swamp Thing, Deadman, Zatanna, el Dorado or The Spectre just to name a few, and fans pointed out it seemed unlikely that so many heroes would have been active at the same time without teaming up. Plus, villains like the Injustice Gang and Prometheus are referenced, who usually require the entire League to take down. Complicating it further, the Arkhamverse was lauded for reconstructingSuperman Stays Out of Gotham (Since each game takes place in less than a night, Batman has no time to ask for help) so retconning the League as having never existed seems rather contradictory to the prior games’ worldbuilding.
