I’ve posted some mean answers in the past, so I may share some insights:
Someone had a bad day. Maybe a client berated them, maybe they had a falling out with a family member, maybe they stepped into a dog poop on a rainy day and their umbrella got blown out before a passerby burned their hand with a lit cigarette (too specific? yeah, well…)
Someone decided to self-medicate (booze and weed seem to be popular choices)
Someone forgot to take their meds (high blood pressure can do it, the flu will do it, insomnia or psych meds will do it more)
Someone got a series of the aforementioned.
Generally: people post mean answers when their sense of empathy is either inexistent, or beaten into oblivion.
Once there are enough people in a place, the chance of encountering at least one person in one of those situations, quickly grows to 100%. If the place doesn’t actively discourage that kind of behavior because “engagement”… then you get the likes of Reddit, Twitter, Facebook, and similar.