Split - The transphobic horror movie

Recently, the thanks to the internet, wellknown horror movie Split. Curious after all the drama, I of course went to watch it. Besides, my love for horror movies is well known.
I was surprised. The movie, surrounding a person with dissociative personality disorder who kidnaps three girls, shows just as most classical horror movies, the dangers of a sick brain not properly treated, with adding a slight bit of supernatural senses into the mix. The movie was excellent, and I consider it to be a redeeming of M. Night Shyamalan. I would highly recommend that anyonw who considers themselves to be a fan of the horror genre watch this movie, since it isn't as much as of a "typical" horror movie, and it features a few new approaces to the scaring stigma of horror movies.
Now the reason the internet (*cough* tumblr *cough*) is so upset about this movie is, wait for it... One of the antagonists personalities is a woman. Shown in the trailer with high heels and a necklace, this personality within Kevin, angered bloggers and other participants of social media, because "it is transphobic". I don't understand where this is coming from, when during the movie it is very evident that having a female personality in Kevin's body is not the reason for the horrors surrounding him. Furthermore, they were accusing the movie of "portraying people with mental disorders as dangerous". For those of you who were actually offended by this, look up any real killer, and you will find that they have had psychological issues. Look up any horror movie, and most likely, it's featuring someone with a mental disorder, so why are you freaking out about this particular movie?
I, myself, stand with trans people, and I genuinely believe that not all people with mental disorders are dangerous, and if treated correctly, even the more serious disorders exisiting at the moment, can be a lot less dangerous.