Major spoilers ahead for Episode 12 of Arrow Season 6, called "All for Nothing."
Arrow has raised the stakes in some crazy ways in Season 6. The second half of the season kicked off with Team Arrow split in two just as a team of villains led by Cayden James took Star City hostage. The only thing the good guys had going for them was the fact that Vigilante was actually a double agent and leaking information to his lady love Dinah, Curtis, and Rene on Team Arrow 2.0. In "All for Nothing," it seemed at first like Vigilante could become the next member of Team Arrow 2.0, and even Oliver kinda sorta gave his seal of approval to Vigilante's involvement. Unfortunately for the good guys, Vigilante was murdered before the end of the episode, and the rest of Season 6 will be quite different because of it.
Here's what happened. Both Team Arrows needed information that Cayden James had on his servers, and Vigilante offered to get it for them. Given that Vigilante is valuable for his fighting skills rather than his technology smarts, James was understandably suspicious when he caught Vigilante checking out his technology. For a while, it seemed that some quick thinking from Felicity and a solid performance from Vigilante was enough to convince James that he was loyal, but "All for Nothing" proved that James could not be so easily fooled. He sprung a trap that sent Team Arrow 1.0 -- that is, Oliver, Diggle, and Felicity -- off on a wild bomb chase that yielded no results and lured Team Arrow 2.0 on a rescue mission to save Vigilante.
Sadly for Dinah, she reunited with Vigilante only for a few moments before they realized that they had fallen right into Cayden James' trap. James strode into the room with Black Siren by his side. Dinah was trapped under a piece of debris and only able to watch as Black Siren first impaled Vigilante on a stake sticking out of the wall and then used her Canary Cry to murder him. Poor Dinah had to watch the man she loved die a brutal death, and it's not something that she'll be able to shake off right away, especially since she learned that the data Vigilante had risked his life and ultimately died for didn't even lead Team Arrow 1.0 to the bomb. She believed that he had died for nothing.
Not one to take things lying down, Dinah reacted to Vigilante's death by swearing to avenge him. She told Oliver that she intends to pick off every single member of Cayden James' evil syndicate and kill them, which... honestly, doesn't sound that bad by Arrow standards. It wasn't all that long ago that Oliver was killing everybody who got in his way, and he hasn't shied away from killing his supervillains if he seemed them too dangerous to live. He even killed his own doppelganger in the "Crisis On Earth-X" crossover.
All things considered, maybe he should just get out of Dinah's way and let her go on her quest for revenge if he doesn't intend to help her. Would it be emotionally healthy for her? Of course not. But it could be for the greater good considering Cayden James had Star City locked down from the rest of the country and is currently fleecing the local government of its money. Whether or not Oliver gets in her way remains to be seen; his expression when she declared she wanted to pick all the bad guys off indicates that he's not going to support her.
Vigilante's death wasn't entirely in vain, despite what Dinah believes. Although the data didn't lead Team Arrow 1.0 to the bomb, it did contain footage of what looked like the Green Arrow killing Cayden James' son, which is the act that motivated Cayden James to go after Oliver in the first place. According to Felicity, however, the video was doctored, and the digital signature indicates that the person who altered the footage was the same person who created the fake image of Oliver as the Green Arrow that came very close to outing him earlier in Season 6. This means that Cayden James may have a second traitor within his ranks.
On the one hand, a second traitor could mean another unexpected ally for Team Arrow. On the other hand, we can bet that this second traitor has his own vendetta in mind given what he's helping James accomplish. Also, if the traitor does somehow turn out to be helpful to the good guys, he could very well end up dead as part of Dinah's mad quest for revenge. Vigilante's death has taken the good guys' man inside out of the game, turned Dinah into a wild card, seemingly wiped out Quentin and Thea's hope that Black Siren could be redeemed, and did nothing to help find the bomb. Hopefully the data that he pilfered will have its uses, even if not bomb-related. We'll have to wait and see.