Although it may seem sudden for the character "flip", I think two things are at play: first, MC still thinks the opponent is an other worlder like him, which would explain MC's desire to remove him. Second, the onslaught of enemies may have triggered something in him from his 22 years of...
What kind of moron just stands there with a quizzical expression on her face, while her whole squad gets wiped out?
Lmao maybe expecting even a hint of realism is asking too much from this manga? 😂