By skyforbes 
Is the main mentor figure to Will this season. The last time Will had a mentor figure in a season he was a main character in was Season 2 with Bob. Bob got cooked.
Season 2 had a theme of Will being a spy for the Mind Flayer and this season everyone behind the scenes have been yapping about a big betrayal, a major theme being traitor and they said to re-watch season 2s episode The spy for this season.
Wills connection to the Hive Mind indirectly played a part in Bobs death. This season Vecna is going to take advantage of Will through the Hive Mind as well as seen from the trailers. Things go apocalyptically bad for the heroes from the looks of it and someone has to die for there to be a meaningful impact on the plot.
The Duffers most definitely are not killing off the main core cast. Steve is too obvious of a death. Robin is the perfect character to die and get the maximum emotional impact while fitting into the story nicely and being an unexpected gutpunch.
Robins whole speech is SO intensely "this character will die" coded.
We do not see much of Robin in the remaining trailer shots