[Movie] Avengers: Doomsday | Steve Rogers is...
The "bad guy", and it comes down to the Trolley Problem.
The trailer introduces Steve Rogers in a very eerie way: Thor has a look full of misgivings, warning lights are flashing red, and the music doesn't switch to a hopeful note. This clashes with Steve's friendly smile and Mjolnir confirming his identity.
So what's going on?
Before Thor or anyone else in the Avengers cast encounter Steve, I think Dr. Doom contacted Steve and explained his solution to the crisis, a plan that would save the greatest number of lives. Unfortunately, that excludes everyone in the MCU and X-Men timelines. Dr. Doom may have also arranged it so Steve and his family will be saved.
Mjolnir still responds to Steve because he's working to save as many people as possible; Doom's plan is \*objectively\* the best solution. But that still puts him at odds with the rest of the Avengers.
In the end, I think the Avengers will succeed in disrupting Doom's plans, leading to Doom attempting to salvage the situation and create Battle World. At the conclusion of Secret Wars, the Avengers will have to admit Doom was right, and they'll have to make their own "impossible choice" -- many will lose their lives, their very universe, in crafting a new one. Steve Rogers will survive, and he'll have to carry the guilt of his choices with him.