The Human Game: A Shrine for a Fake God

The choice seems obvious. So, the rationale/wording may be crucial here.

Abigail: Put your trust in a monster like that.

The doors are just a confirmation, really. To what? Let's see:

• How big does Cancel's medallion look like? About an inch? However, in Kate's hand it it's more like four inches. That would make him more-or-less four times larger... about 21'8" - almost seven hundred meters tall. And yes, they are roughly the same size.
  
• Cancel's sheer size would partially explain the mirror statue - it's mirror is sized and positioned for him, and Banksy knows that. But closer inspection of the facility so far implies Cancel would not be able to approach it, as all of the shown passageways are much smaller! Even without that detail this raises the question - was the facility built around him? Had he grown into it? Did he acquiesce to it? Did he plan for it?
  
• Of course, Cancel isn't the only suspect. Or maybe he is one of these angels.

So yeah. The doors are really just that. Doors.