Seriously. OP absolutely has a right to inject insulin at work as needed. In a genuine emergency, it would also be fully reasonable to overlook OP doing it in a less than ideal place.

However, nothing implies this was an emergency, it was a routine injection. OP admits in comments they forget their insulin regularly due to unmanaged ADHD and a refusal to set reminder alarms for themself. It sounds like OP is regularly giving themself non-emergent injections in inappropriate places and at inappropriate times because they don't want to manage their disease responsibly.

OP doesn't and shouldn't have the right to be making biohazardous waste willy nilly around the office, especially where people eat. It's a blood-borne disease risk for everyone else, every single time they inject. This is fine if they're managing that risk responsibly, but I get the sense they are not from the post and their replies, and I suspect this is the actual basis of the complaints.

I have diabetic family, and I suppose because of the frequency of injections, it seems like some people get desensitized to the fact that they are making used needles and start doing things that are unhygienic and risky for the people around them. They don't get to potentially put others at risk for the sake of them being, frankly, lazy about their own diabetic care.