Answer: This is a (relatively) small part of a much larger and very complicated story and details around this specific part of it are still confused, but I'll try to keep it concise.
Disclaimer: I absolutely have a horse in this race but I'll do my best to keep my answer as unbiased as possible.
The twitter user Atherist in the link posted worked briefly as a storyboard artist for Magiswords (of which Kyle Carrozza was the creator). They had an alternate twitter account where they posted NSFW artwork including a slash piece between two Invader Zim characters. Kyle was alerted to the existence of this alternate account sometime during 2021 and broadcast its contents to he and Atherist's mutual friends and former coworkers.
As for why people are mad about this now, two years later; recently Atherist posted what is effectively a suicide note to Twitter. In it they state that their life is beyond saving due to lack of health insurance, lack medical care for their disability, and inability to find work. Some people are angry at Kyle because they believe his past actions contributed to Atherist's situation.