This needs no explanation really. Some people are just so negative or bring no type of growth to your situation. The luck of these people can be infectious, so while you may be the happy go lucky top performer that you are, their negativity can tend to rub off on you in various ways.
When you cut ties, do so with no announcement. Just be done, and finished with the negativity. Don’t give any chances or warning.
Flee the situation for your own well-being. The culprits in these scenarios are usually people you will know.