This was a private communication between Bill and Sue and you only saw it because she was sharing her screen.
Have you ever made such a comment like that yourself to someone else privately? Now did he do it on purpose, knowing that you would see it? Short of a confession from Bill, that's an impossible thing to say.
But either way, having filed a complaint with HR will ensure that he can't make that kind of mistake a second time (intentionally or not). So inAnd considering that sense, you madethis was the right callonly incident with this person, by contactingcomplaining to HR, you did all you could do on that front.
Would my only option be to quit at that point?
Are you serious?
Do you have such a thin skin that you will quit over a single incident like this? I'm sorry, but I think you're overreacting.
Sometimes, meetings run over and participants get impatient. And a comment made "to finish the meeting already" has nothing to do with the current speaker themself.
Even the laughing, there is no evidence they were laughing at you. They might have just been laughing at Bill for being such an idiot with instant messaging, and/or for acting like such an impatient child in the first place.
What about you, would you have laughed if the situation was reversed? What if Sue had received a private lovey-dovey instant message from her boyfriend/husband instead? Would you have laughed then?
Laughing is also a way to release tension. And yes, by stopping mid-sentence and shutting your laptop screen the way you did, you did contribute to the increased tension Bill's "mistake" generated.