I don't think it's HTGAWM because of reasons already said.
No one is safe in Whedonverse but I'm not sure who they could even kill in Agent's of Shield. Hunter and Bobbie are safe because of spin off, Coulson is safe as director, Fitz and Simmons have both gone through enough this season I don't see them killing off one or the other at the moment, and Daisy/Skye is way too important to the story and the entire MCU to kill off.
We already know someone of importance will die on Arrow based on the last episodes Flash-foward but the person speculated
Captain Lance
I don't think is a big enough deal for them to make an article about.
I don't watch the other shows but I think Agent's of SHIELD, Arrow, and HTGAWM are out of the running for this.