Robin is trapped in some cages, when Slade and you walk up to him as Slade pulls a gun to Robin

"Heheh.. What are your last words, Robin? Or are you going to serve me and my son forever? Your choice."

Slade walks closer to Robin, gun still in hand

Robin: "N..No.."

"No?"

Slade laughs

"Fine then. Your loss."

Slade shoots at Robin, killing him, and then exploding the Teen Titans HQ, you escape with your dad, but the Teen Titans HQ sinks into the water, and the Teen Titans drown, never to be seen again.