MC knew that the B ranker could only use his powers around a dungeon, so his family was safe, and the girl was even safer than him as her stats are likely even higher.
I really don't like how we are discovering MC power here and there, without a decent explanation, I didn't expect him to be able to forcefully monsterize random animals.
Seeing how MC run, literally run, to help the bullies wannabe rapists of the girl he cares so much I'm guessing he had no idea something was going to come out of the portal, he is clearly a toxic hero that tries to save anybody, even if evil, that idea is even strenghtened from how he wanted to use the video to keep the B rank low or ruin him, that really makes it look like the author had some other idea of how the story would continue but readers bad comments about the serie pushed the death sentence.
I presume the B ranker could have defeated that monster normally, but being tired and beat + nt realizing it was behind him screwed him up.