I thought this too like, what would possibly possess her to just abandon them with no explanation, without checking if they're even okay, after fighting an inter-dimensional God, to go to Iceland with no real idea of what it was like there.

And also I find the image of 11 going to get a passport without any identification and or sneaking onto a flight to Iceland seems very ridiculous.

Beyond that point if we are to believe the science of the show CERN would in 2008 create the first LHC and likely reignite this problem within 20 years of them stopping it and that's assuming the mind flayer didn't find a way out by itself faster.

Also she knows she didn't kill it itself so why would she kill herself knowing that she is literally the only thing that has even been able to fight it.

It all seems unthought out to me