Is it acceptable to leave an adjunct teaching position 10 days before classes begin?

I think it's generally agreed that it is unwise to tell an employer that you might leave, that nobody does this, and that nobody expects anyone to. You only tell them when you are definitely leaving, which you did as soon as you knew. You haven't done anything inappropriate.

Your old department head is surely not thrilled about having to fill your classes with 10 days to go, but it's their job to handle such contingencies, just as if you had been hit by the proverbial bus. You weren't under contract and so you're a free agent. I probably wouldn't expect them to hire you as an adjunct again, but it seems unlikely you'd want them to anyway.

Firstly, in academia this is an expected situation, although they are not happy, they just need to figure this out. It is very natural for you to accept a much better opportunity, especially when you are switching to a full-time position. You are not expected to tell them unless you have a definite offer and you have made arrangements to take it.

That being said, you should try to make it easier for them as much as you can. Do you know anyone that could take over - within the institution or from outside- Is there a TA that could take over, or can you ask some other faculty working on similar topics? Maybe you can provide them your teaching materials? Can you ask your new employer whether you can 1) start at a later date? 2) teach at the former institute at the same time (if location/time allows or remotely).