Warning: spoilers for Dark Crisis #1 are ahead!
Leader of the Deathstroke dating back to the Judas Contract story arc.
This is the same arc that reveals the true intentions of Terra, a character who s the Titans as a mole secretly working for Deathstroke. Prior to the reveal, she begins a romantic partnership with Beast Boy. For Terra, it is merely a ruse to earn his trust, but for Beast Boy, he had actually fallen in love, turning that betrayal into an even sharper sting for him. Surprisingly enough, their on-off relationship has more so focused on Beast Boy trying to sway Terra to embrace her more heroic qualities rather than directly feuding with her mentor.
This relationship between Beast Boy and Deathstroke (or lack thereof) reaches a turning point in the conclusion of Dark Crisis #1 by Joshua Williamson and Daniel Sampere. It is here where Deathstroke launches a sudden attack on Titans Tower, culminating in the hitman putting a bullet through Beast Boy's head. This brief encounter showcases why Beast Boy has been Deathstroke's real archnemesis this whole time. Not only because of their overlooked history but because killing Beast Boy is the most significant blow that Deathstroke could ever give to the Titans as a whole.
Although this is a brief exchange, it offers audiences the most significant exchange between Deathstroke and Beast Boy since perhaps kill now being in Deathstroke's hands, who does what Beast Boy couldn't in a moment that is much weightier for those readers who are familiar with their history. Even those unfamiliar will feel the weight and impact this death surely will have on the Titans moving forward.
Deathstroke's villainy is most commonly associated with the Titans and therefore he is most often written to antagonize those heroes, but he's never done something as dastardly as this. The reason why killing Beast Boy is so significant is because Beast Boy has always been associated as the heart and soul of the Titans. He's been depicted as that uplifting, optimistic spirit to remain hopeful in hopeless situations and get the Titans back on their feet once they've had the legs cut from under them, like in this instance of the destruction of Titans Tower. Destroying the heart of the Titans takes away its beating heart. The DC Universe has already seen the death of the Justice League and by killing Beast Boy, Deathstroke has spelled the unofficial death of the Titans.
Dark Crisis #1 is available now from DC Comics.