I find the most straightforward response is, "would you do/say that when you're having a bad day? Because I don't think I would"

The state of your mental health is not necessarily your fault, but it is your responsibility. It's ok to be having a bad day, week, or year but it's not ok to hurt other people because of that.

"Well in that case, he needs to learn better coping skills."

At my absolute worst, on the worst possible day, I always start by apologizing for being frustrated.

Maybe they’re just an asshole.

“they could just be having a bad day!!”

It doesn't give them the right to make sure I do too.

"So could I. Worst case, they are the last straw, pushing me do something that can never be taken back."

I think it's true, but it's definitely not helpful in the moment. You can treat someone who isn't nice with a baseline amount of respect, but if their behaviour is bad out doesn't always matter how their day is going, the outcome can be the same