Even if you see it as being an insult up for anyone, why, why would you tweet that? To or about anyone?
I said that it wasn't an insult up for anyone but one used almost exclusively for women.
As for the why, given the context within which these Twitter exchanges occurred, I can think of a few choice words that I would use about some people - not to annoy the other person, but to express my anger and relieve my stress and tension. Which is an incredibly well known phenomenon, so much so that we have research showing that swearing measurably reduces pain, stress, blood pressure etc.
It is simply completely inaccurate to claim that every time we use an insult directed at someone we seek to cause annoyance or anxiety. Many times we don't think about how the other person will respond to the insult at all, we just let rip. Without considering the other person's feelings. Mumsnet is full of threads about that.