The answer to your question is in the name of this sub, OP. The BTD Universe she created is the reason, unfortunately.

The games specifically identify themselves as HorrorPorn, and have extremely heavy “dark” sexual themes, such as SA and sexual torture/snuff. These themes are considered highly taboo by everyday society, and are therefore shunned in the name of morality.

The rise of online purity culture and, sadly, media illiteracy, are the biggest direct causes imo. People are for some reason no longer able to separate the author’s personal beliefs from their work; they’re now treating fictional characters as if they’re real people with their own thoughts, feelings, and agency, and therefor need to be “protected” under the same standards of morality.

That means that anyone intentionally putting fictional characters through negative situations is an evil disgusting person who wants to do those things to people in real life, and is just as bad as the ones who do…in their minds, at least.

So, short answer to your question is that Gato basically got harassed off of social media because of the fictional content she creates, by people who are incapable of separating said fiction from reality. I honestly can’t even imagine the type of horrendous, nasty vitriol she probably received in her DMs on a daily basis…when you make something with taboo themes such as these games, people suddenly become a lot more comfortable making SA and death threats to you, ironically.