Gary Johnson has a substantially better chance to win the general election than Trump has to win Illinois. Think about that. If you vote in Illinois, your vote for Trump will NOT help one iota in keeping Hillary from the White House.
Not one iota.
The same is true if you are a Hillary voter trying to keep Trump out of office. Hillary will win Illinois in a landslide. Trump has no chance in hell.
What state do you live in? Take a look and see if the same applies to your state. Unless you live in one of the ten "Purple" states, you are in the same boat.
Vote for something, not against someone.