In a two party system, voting third party will only ever result in your vote being thrown out. It is taking a stand against reality for the sake of your own personal idealism.
A lot of left-aligned voters learned that lesson the hard way in 2016. If you didn't learn the lesson, you either weren't paying attention or your idealism is more important to you than the actual outcome of the vote.
Doing that willingly, despite (or maybe because) the rest of your side screaming at you not to fuck this up again, is absolutely a level of betrayal.