Nikubou Wa Itsuka Uragiru TS De (“Eventual Betrayal Of Your Cock After Gender Swap”) [English], by Reiha
The usual magical girl hentai fate: facing off against too strong an opponent, betrayed, she is tentacle raped, mindbroken, ends up a reproductive puppet.
This share’s magical girl hentai fate: a man is turned against his will into a magical girl, the people he/she saves aren’t grateful, the evil female villain convinces her to switch sides, and now they’ve become buddies they become the (sexual, of course) mighty evil rulers of Japan ┐( ̄ヮ ̄)┌
Don’t tell me you had seen it coming, lol.
The drawings are pretty good, with hardly noticeable censorship most of the time, maybe – if my summary didn’t scare the shit out of you ^^;; – that’s worth a shot, check it out?
Espeon and Desudesu are behind this release, thank you!
By the same artist, I also share Shibatsu Chokugeki Kawaii Ko Ni Sarete Yogarasareru Ore (that one is also a dark gender bender story) and Yakujin-sama No Yasashiki Netori.
And now, the download & gallery links 
Open the Complete Pictures Gallery
Download the Free Hentai Manga in a Zip file
(20 MB, 22 pictures, English)
Or you can also use the Alternative:
Zip Mirror #1 – or – Zip Mirror #2 – or – Zip Mirror #3
I don’t usually go after gender-bender stories, but based on the synopsis, I gave it a go. By The Close, I Agree that the population of Japan in this story, and that Mascot brought this upon themselves:

Hmm. It either needs more plot or less plot. This is like the anti-Goldilocks level of plot: JUST WRONG. Heck, I thought the magical girl would go Futa at the end and bang the bad girl, that would have been more entertaining!
IMO that’s one problem with single chapter stories– 20 pages are usually not enough pages to deliver on both plot and erotic content.
These two girls look somewhat familiar though– they look like older versions of Vivio and Lutecia from Nanoha Strikers. LOL
so….she used to defeat them with explosions, the monsters are basically genetically modified humans, and since she turned evil no magical girl will come save them…
Thankfully we still have Mr. Thermonuclear to save our day,a couple mirvs will do the trick(at the cost of every japanesse life, but at that point lets be honest, its a mercy kill while denying enemy ressources) unless it needs to be a magical explosion…
then we’re all fucked…then monstrified or dead.
Or as an absolute last resort, we could apply THIS:
http://memedad.com/memes/1569066.jpg
The game of Risk, hentai style.