I feel like Raddy was the first to die, he was the leader and with his death it could bring terror across whatever world they were in and keep them apart and undefended.
I also feel like Blue was aware of all of this, possibly trying to prevent it, he's the most aware of all, He's also confirmed to be a good guy, maybe managing to hide away the survivors for the time being except...you know...White/Wenda.
There are multiple things that could have happened with Durple and Simon, either they were corrupted somehow, or they went crazy along with Wenda, killing the others.
The current survivors are Brud (Somehow), Wenda (Insane), Gray, Jevin/Blue, Fun bot and Mr. Fun (Traumatized or just corrupted in some way).
I also wonder wth happened to Mr. Tree, it's kind of strange how they just disappeared, maybe since it's a tree it was affected by the change in environment or something.