The little fluffy mongoose was sick and tired of his species being able to be female. Every time he saw a female, or even thought about one made him sad, depressed and always made him feel like he should give up on life.
One of his neighbors in the swamp was a female Zangoose. He hated her with all his furry heart because she was a female.
He wanted her to just die along with her whole gender in his species. He knew being a fluffy mongoose would allow his species to live without a female, but even so, he wouldn't need to make love with one of his species.
He frequently browses Deviantart, and Tumblr to cope with his feelings for the females due to those sites disliking female Zangoose to an extent that would only match his utter hatred for them.
These sites always made him feel a little better, of course until he saw a female, or someone referring his species, or a single Zangoose with female pronouns.

He knew how manly his species was, so there was no need for females to exist.
He always wished that his species would be like that of Chansey, or Kanghaskhan having a 100% gender rate. He always wonders why the lord Game Freak never made him that way. He wouldn't accept a lowered female rate of his gender either.
The Zangoose went outside on his furry knees, and prayed to GF once again that his species would eventually become 100% male, chanting "P-Please.. Game Freak.. Please make my species 100% male.. We're too manly to be a female, and we can live on our own without a female.. Please!". He prayed hard, so hard that the fur on his chin started to fluff up. The fluffy mongoose was so determined to have his species 100% male. He was ever so determined that he began to take extreme lengths in doing so, involving killing all the females.
He talked to her male Zangoose friends, and herded them all up. Strangely enough, they all agreed that female Zangoose shouldn't exist.

All five of the Zangoose went back into the swamp to kill the female.