If "mentor" describes someone that you learn important lessons from, and you are intentionally developing yourself towards their example ...
What would an appropriate word be for someone that demonstrates to you what not to do, and that you are developing away from their example?
The closest example dichotomy I can think of is hero v, villain.
Edit- Perhaps a better word for comparison is "role model"- in that you may not know them personally, but you are actively pursuing their example. The antonym I'm looking for describes someone who you do not want to be like- however this does not necessarily mean that they are a bad person, so these terms don't seem to fit;