Is this just being spiteful?

There is someone at my work that I am finding unbearable to be around. She talks to me like I am thick. I can’t complain about because to literally everyone else, the sun shines out of her arse. If she has a day off, everyone is like ‘how will we cope without her’.
Anyway, I have been thinking for about 4 or 5 months now, to send an email to work as an anonymous client. Just saying that she has noticed how this person talks to the other members of staff, and that when they go in they feel uncomfortable to watch how she is other people. That kind of thing.
What is stopping me doing this is guilt, if this is a mean thing to do?