As a rule, give members the benefit of the doubt.
They’re not bad people, they’re just having bad days. They didn’t sleep well last night, they just came out of a major relationship, they’re stressed about work, and broken their ankle yesterday.
Anyone of these could explain why they lashed out at another member in that message.
In the broad realm of possibilities which may explain an awful post, an external cause is rather likely. If you assume the best and drop the member a personal note to check they’re ok, you’re far more likely to bring them back into the fold (compared with punishing them with an instant ban).
The odds on any member being so bad a person they need to be instantly removed and punished is slim. External causes are far more likely than internal attributes.
The flip side of this is you don’t get the benefit of the doubt. You’re judged by your last contribution. If you’re having a bad day and lash out, even just once, that’s the ball game. A single provocative, aggressive, or cynical post is how you’re judged as a person and you’ll struggle to recover from it.
If you didn’t sleep well last night, are feeling angry and aggressive, it’s probably best to summon the strength to step away from the keyboard (while assuming your members don’t).