If the type of behavior between "Case" and "Note" is “Cascade None” on Assign, it happens 2 scenarios:
1. If the user is the owner of the case, he can delete all the notes, independent if he writes it or not;
2. If the user is not the owner of the case, he is only able to delete his notes.

Suggestion:
When a user is the owner of the case, he shouldn’t delete someone else's notes when he has the permission to only delete his notes.