The latest episode of Titans hints that Deathstroke may have killed an original member of the team. The second season of DC Universe's flagship TV series has had a strong start; the Titans season 2 premiere wrapped up the Trigon plot, and since then the show has pivoted to focus on Deathstroke and Doctor Light.
Titans has pulled this off by introducing a smart retcon; it turns out the kids are the second generation of Titans. The original team consisted of Dick Grayson's Robin, Wonder Girl, Hawk and Dove, and at least a couple of others; Speedy (Roy Harper) and Aqualad have both been name-dropped, suggesting they were part of the group as well. Unfortunately, the Titans ended in tragedy, with the group disbanded. Dick Grayson is currently attempting to conceal his past from his wards, which allows for a nice dramatic reveal over the course of the season.
Titans season 2, episode 3 hints that the team may have disbanded after one of their own was killed by Deathstroke. It confirms that the Titans wound up in a grudge match against Deathstroke, and that Dick Grayson made some poor judgment calls. When it was all over, the Titans thought Deathstroke was dead, but their victory had come at a cost; Donna mentions that there are ghosts in Titans Tower, and the tone of her voice implies she means that literally. Dick blamed himself, and it's possible some of the others did too, and the Titans broke up.
But who died in the battle between the Titans and Deathstroke? It's difficult to say, given the show is essentially making up its own history, but the most likely candidate is surely Deathstroke's own son, Jericho. Created by Marv Wolfman and George Perez, Jericho was a sensitive, artistic kid who joined up with the Titans. A mute who communicated through visuals, without the use of thought balloons, Jericho helped the Titans when he learned they'd been targeted by his father. He became a committed member of the team, but tragically was possessed by demons, and Deathstroke was forced to kill him.
Titans has already confirmed that Jericho existed in the DC Universe, and that he'd been killed by Deathstroke; Rose blamed her father for her brother's death, and that's the whole reason she's in San Francisco hunting down the Terminator. If Jericho was a member of the Titans, then events must have played out a little differently, because in the comics he was possessed because his close friendship to Raven left him exposed to the power of Trigon. Titans is clearly switching that history up, with Rose becoming friends with Raven rather than Jericho.
Of course, while Jericho is the most likely candidate, the truth is that it could have been anyone. There have been almost-innumerable members of the Titans in the comics, and the show is blazing its own trail, meaning anyone could be retconned as a member of the original team. Right now, the only one who can be discounted is Speedy, who's clearly alive in the present day and gave Donna a phone call in Titans season 2, episode 2. Even Aqualad can't be ruled out; while he'll appear this season, that could be in flashback form. Only time will tell what DC has planned.