WARNING: The following contains spoilers for Extreme Carnage Alpha by Phillip Kennedy Johnson, Manuel Garcia, Cam Smith, Marc Deering, Roberto Poggi, Guru-eFX and VC's Travis Lanham, on sale now.
The Marvel Universe is still reeling from the events of King in Black's symbiote invasion. The worldwide crisis left wreckage everywhere, even in the confines of Iron Man's lab. As the Avengers were trying to figure out how to take on the symbiote god Knull, Tony Stark merged one of his Extremis suits with a symbiote to create a new being.
And in Extreme Carnage: Alpha #1, the living suit of Extrembiote armor lives on as an uncontrolled, snarling reminder of the symbiote threat.
Ever since Eddie Brock ascended to godhood and gave the other symbiotes in his Hive unprecedented freedom, they have mostly flown under the radar and carved out simple, ordinary lives for themselves. Of course, Flash Thompson has become intimately aware of the problem this poses, especially with the general populace sharing a dangerously growing anti-alien sentiment. Now that Carnage has made his own gruesome return, Flash has more to worry about, though he isn't alone in those concerns. Tony Stark is also aware of the situation brewing and reaches out to Flash for a consultation of sorts.
Unfortunately for Iron Man, Thompson points out yet a serious problem with the Extrembiote armor. To potentially save a dying Eddie Brock's life during King in Black, Tony rewrote a symbiote's DNA with a version of the Extremis Virus. While the symbiote didn't save Eddie, it did provide Tony with a powerful weapon against Knull's army. It has also left him with what is effectively a prisoner as Flash explains. The newly dubbed Extrembiote might be bonded to the suit, but it is still a sentient being that is presumably aware of its captivity.
While this is an ethical nightmare, it could also be the birth of Marvel's next massive threat. Since it has free will, the symbiote could easily turn against Tony Stark and use the suit's powers for a variety of nightmarish revenge scenarios.
Given his status as one of Marvel's most forward-thinking geniuses, Stark should've thought of this already, especially since it essentially already happened to him once before.
In 2000, Iron Man #26, by Joe Quesada and Sean Chen, saw the creation of one of Iron Man's most formidable villains, a corrupted Iron Man suit. After a lighting strike hit Stark during a fight with the villain Whiplash, the armor that Tony was wearing at the time became sentient, and this new creation quickly became enamored with its creator. The situation took a horrifying turn when the armor murdered Whiplash in cold blood. When Tony tried to shut it down, the armor turned on him. Tony was kidnapped, dragged to a deserted island and tortured for days on end by the sentient Iron Man suit. When the strain led to a heart attack, the armor found some semblance of empathy and saved Tony's life at the expense of its own, though this hardly changed the terror it had wrought in the meantime.
Somehow, Tony is oblivious to the fact that another aggressive, sentient suit of armor could be problematic, even after having already lived through that particular worst case scenario once already. Not only could the Extrembiote very easily and understandably turn on Iron Man at any moment; it could even be conscripted into Carnage's hive. If this were to happen, then Stark's symbiote Iron Man suit could be one of the deadliest symbiotes to ever walk the face of the Earth in the Marvel Universe.