Spoilers ahead for Episode 20 of Arrow Season 7, called "Confessions."
Roy Harper has been a significant part of Arrow in Season 7, but always in the super grim flash-forwards that have him teaming up with the next generation of Star City heroes. He returned in the present in "Confessions," and he was a big help to Team Arrow as they worked to stop a terrorist attack on the city... or so it seemed.
While Team Arrow did thwart any terrorist attacks on the city, they fell into a trap at least partially due to Arsenal, and his return revealed a big death. Star City may be doomed, and what happened to Roy may help explain the state he was in when the flashbacks kicked off in the beginning of Season 7. Let's start with the big death.
The last Arrow revealed about Roy's actions prior to the flash-forwards, he was working with Thea and Nyssa to destroy the Lazarus Pits that caused so much pain. Sure, resurrections eventually worked out in cases like Sara and Thea, but it was a long and bloody road before they were all the way recovered. According to Roy, the mission was going well, and they were honing in on more Pits to destroy.
What Roy left out is that something went very wrong on one of their missions, and it wasn't until after he beat two innocent guards to death by hand in a murderous rage that Oliver figured out what had happened. Roy had died while working with Thea and Nyssa to destroy the Lazarus Pits.
Yes, Roy ultimately came clean and explained that the Thanatos Guild had them surrounded, and when he tried to get Thea to safety, he was hit and killed by an arrow, of all things. Roy died, and he was brought back by the waters of the Pits they were working so hard to destroy.
He didn't have any proof about why it was affecting him the way that it was, but he theorized that the waters interacted with what the Mirakuru did to him way back in Season 2, leaving him even worse off and prone to worse rages than others who were dipped in the Pits.
Roy was responsible for beating those men to death, but Team Arrow wasn't willing to let him go down for it, and the whole crew engaged in a big conspiracy to convince the SCPD that Emiko was the guilty party. Oliver, Diggle, Felicity, Rene, Roy, and Dinah all lied in a coordinated effort to paint Emiko as the murderer with Roy as a guy who just happened to lend a hand to Team Arrow.
Their saving grace was that somebody had removed all security footage of the incident. Suspiciously, Felicity was not the one to do it. On the whole, it was one of the shadier things that Team Arrow has done, and it's definitely going to come back to hurt them.
Oliver came to the discovery at the end of the episode that the entire affair had been orchestrated to guarantee the destruction of Team Arrow in the eyes of Star City. Orchestrated by whom? Emiko, of course.
I'd say that Emiko spinoff idea should be pretty dead by now! Emiko was the one to remove the security footage, and she sent it right to the desk of the SCPD detective who had it in for the vigilantes of Star City (and who I totally thought was the one framing Team Arrow for a solid 75% of the episode).
The SCPD has evidence not only that Arsenal beat two men to death, but that all of their sanctioned vigilantes -- including Captain Dinah Lance -- had worked together to convince the authorities that another person was responsible. They did the framing for murder.
As if that's not enough, Emiko also dropped the bombshell on Oliver that she'd had to chance to warn their father about the bomb that would destroy the Queen's Gambit, but chose not to as a punishment for Robert's misdeeds against her and her mother. Then she dropped the bombshell that she had worked to ruin the vigilantes' reputations in Star City, when they had made so much progress. Oh, and then she literally dropped the roof on Oliver. Oh, family.
So, Roy died and was brought back, leading to him beating two innocent men to death, and likely leading him to self-exile on Lian Yu. Meanwhile, Team Arrow may be doomed in the eyes of Star City, and now that their identities are public record, they could all go down.
Well, at least Felicity will presumably make it on the run to give birth to baby Mia in relative safety. Oliver too will be free enough to be with his wife when Mia is born. What happens to him afterward remains a mystery.