Slade is the arch-nemesis of the Teen Titans, who wants them destroyed for his own unknown reasons, and is the main antagonist in seasons 1 and He also has a focus on the team's leader, Robin, making him somewhat of an arch-enemy for Robin.
http://minecraftstream.com/minecraft-episodes/nightwing-returns-to-the-teen-titans-brother-bloods-revenge-ep-1-minecraft-roleplay/ - Nightwing Returns to the Teen Titans! Brother Blood's Revenge Ep.1 (Minecraft Roleplay) Minecraft Roleplay |Brother Blood’s Revenge | Minecraft Teen Titans Season 5 Episode 1 | Nightwing returns to the Teen Titans after defeating Slade with the help of his new friends from Young Justice! Don’t forget to subscribe and leave a Like
Well, SLADE would've just threatened to kill them anyways and SLADE is way to sneaky for Robin to actually know what's going on in fact before SLADE offered Robin to be his apprentice, they had never really seen/talked to each other before.