I have a theory that's probably wrong but it will mess with your heart.
What if Defender Strange was PRETENDING he was going to take America's powers and kill her? What if he realized the monster alone wasn't scarying her enough so he pushed things so America would use her powers and survive? So he started to drain her but slowly, so she believed he would kill her, she would open a portal and escape and he would stay behind to buy some time fighting against the monster since his leg was hurt and he wouldn't able to run anymore?
Another important detail is that Defender Stephen did care about her, he seemed quite sad when draining her and even moments before dying, he used his magic to save her.
Along the movie, it's stated many times by different characters, including Strange that "all Stephens are the same". Well, if that's so when it comes to personality (and knowing Defender isn't corrupted by the DarkHold), our Stephen didn't try to kill America to steal her powers, he didn't even consider that along the movie- so it's possible Defender also never truly considered taking her life, he just meant her believe he betrayed her so America would be angry and not care about his death/missing (since he would sacrifice himself against the monster to buy time for her) he wanted America to feel betrayed and hate him so she wouldn't grieve his loss.


































