Should I go through with my plan to undermine my best friend?
I think you are mental for wanting to sabotage them.
I'm a mentally ill person if you associate me with anyone who's healthy
You should get help then before you just ruin your relationships like this. Friendships should be fulfilling and fun, not full of betrayal and sabotage. Your friend would be willing to help you get better if you let them.
I've got it all under control.
It doesn't matter if you think you've got it under control, this person is your best friend, are they not? Do you really want to hurt your best friend? How would you feel if they did something to hurt you?