This is one where there doesn’t need to be any confusion. Voting for Jill Stein (in a competitive state) is voting for Donald Trump to be president. There isn’t any room to quarrel on this.
There are many voting systems where voting for a third party candidates is a meaningful, effective, and non-distorting exercise in democracy. Unfortunately we don’t have one of those systems. We have what we have.
The idea that it won’t tip the election to Trump is a non-viable argument after Nader in Florida in 2000. Whether his voters gave the election to Bush (probably) or not doesn’t matter literally. The ample evidence that they might have is evidence enough of the potential consequences.
You can go back and make the argument if you’d like to that there wasn’t a ‘dime’s worth of difference’ between Bush and Gore. The words ‘Iraq’ and ‘climate’ head up the long list of differences. You will have an even harder time explaining how there’s not a dime’s worth of difference between Hillary and Donald.
Some have made the argument that if Trump wins, it’s not the fault of people who didn’t vote for Hillary, it’s the fault of the Democratic party for not nominating Bernie. Okay, place the fault wherever you like. Trump will still be sitting in the Oval Office appointing Supreme Court justices.
Finally there are those who come right out and say that they fear Hillary more than Trump. This is at least honest, if not insane. But whether it is said out loud or not, that is the conclusion that Stein voters have reached, and their actions will serve that end.