machine?

Machine in the middle is probably even more accurate than man. Keeps the same acronym too which is nice.

Person is the obvious alternative that is gender neutral

Person in the middle. If you want to replace man with a generic form, use person. Monster is childish, and lazy.

Person is lazier - it doesn't even maintain the acronym. Perhaps if you'd suggested Malefactor in the Middle it would have been a more convincing argument? In general I don't consider 'monster' to be childish - it's often used to refer to sexual preditors, for example. What we're seeing in this line of discussion is that language is mutable and subjective - and for that reason the author's use of words is a justified as any other.

I've got a simple way for you to maintain the acronym...