It's not "helping" the worse evil. It's helping neither.

If third-party voters and non-voters "helped the worse evil" then the worse evil has won every election in recent history in a landslide victory. In reality, removing them from the equation has no impact on which of the major parties gets more votes.

It's only if you assume that a party is entitled to people's votes that the word "help" could possibly be used. And that's not how anything works. No one is entitled to my vote.