Step 1: Finish all of your tasks.
Step 2: Leave
Doing this makes it more fair for the impostor, since immediately leaving removes your tasks from the task bar, meaning that the moment the impostor kills somebody, they're at a disadvantage.
First of all, was your teammate a little jerk who ratted you out for no reason? (If they ratted you out to maintain cover, then don't do this)
Step 1: Stalk them
Step 2: Always close the doors of the room that they're in, trapping them inside as much as possible so that you can annoy them as revenge.
Now, what if they were a good teammate?
Step 1: Fly around the map
Step 2: Close doors of rooms with two people in them to raise suspicion
Step 3: If there are only 3 people left, your teammate and two others, take the opportunity to trap them all in a room so your teammate can have an easy win.