Warning: The following contains SPOILERS for Titans' season 2 finale, "Nightwing."
With DC Universe's Titans season 2 now over, it is fair to say that the show has completely mismanaged the greatest villains from the classic Teen Titans comics. This leaves the show in an uncertain state heading into season 3, though there is a possibility that some of these beloved bad guys may return for more in the future.
The first season of Titans was built around the coming of the inter-dimensional demon Trigon and a team of misfits coming together under the command of Dick Grayson, the first Robin, to try and protect Rachel Roth (aka Raven) from her father and his followers. They failed miserably, falling prey to their own inner darkness and into Trigon's control. In the end, Rachel defeated her father on her own, in a confusing sequence in Titans' season 2 premiere, which ultimately came down to the power of friendship winning out over fear.
Titans season 2 then introduced the character of Dr. Arthur Light, aka Doctor Light. Possessing the ability to control light in all forms, Doctor Light was undoubtedly powerful in terms of raw energy. Yet over the years his portrayal in the comics has ranged from that a cruel and efficient mastermind to a bumbling buffoon. Titans' Doctor Light was definitely the later, being easily overpowered and outwitted by Jason Todd, before ultimately being killed off by Deathstroke for being more of a detriment to his plans than an asset.
It was Titans' handling of Deathstroke, however, which was easily the show's biggest disappointment, largely because of how much the show got right about his character early on. Esai Morales cut a menacing figure as Slade Wilson and the show's writers crafted an origin story for their take on Deathstroke that was the most comics-accurate adaptation in history. Unfortunately, Deathstroke deteriorated as Titans' season 2 progressed, with the show's writing being inconsistent on whether Deathstroke's grudge was focused upon Dick Grayson or the Titans team in general.
In one episode, Deathstroke's plan would seem to involve playing head-games with the team in an effort to drive them apart. In another he seemed to be focused on driving a wedge between Dick Grayson and the team he had built, depriving the former Robin of his family in the same way that Deathstroke felt Dick Grayson had taken away his family. This particular point was rendered utterly nonsensical when it was revealed that Deathstroke had forged a relationship with his estranged daughter Rose after driving the original Titans team apart and that his son Jericho's spirit was still alive in his head.
Ultimately, for all the build up that Deathstroke received, he was unceremoniously stabbed through the heart by Rose within the first ten minutes of Titans' season 2 finale. It was an anti-climactic ending for an unevenly handled character and the greatest Teen Titans villain ever. Given the strength of Slade Wilson's healing factor in the comics, there's a chance he may return in the future. For that matter, Trigon may return from being banished by Raven. In either case, these villains' first outings on Titans were nothing but underwhelming.