Why would you assume that, of all things?
Harrison is out to punish Starfleet, including Marcus. We see exactly what happens, and why - in fact, Kirk figures it out before it happens.
There's nothing in the movie suggesting that Marcus is behind it.
You can make up a story that Marcus for some reason wants to off a bunch of high-ranking Starfleet officers in order to consolidate his power or something - but that doesn't make any logical sense, and there's nothing in the story to hang that on.
True.
Harrison is out to punish Starfleet, including Marcus. We see exactly what happens, and why - in fact, Kirk figures it out before it happens.
There's nothing in the movie suggesting that Marcus is behind it.
You can make up a story that Marcus for some reason wants to off a bunch of high-ranking Starfleet officers in order to consolidate his power or something - but that doesn't make any logical sense, and there's nothing in the story to hang that on.
Further, there's no indication whatever that Marcus attempts to hide the communication later -
In fact he doesn't, Marcus tells everybody about the message during the briefing. In fact the message seems to fall into Khan's plan to get Marcus out into the relative open and kill him.
True.