My only comment on this entire subject is that it depends.
Where I work now, there's strict policy about taking a job with a competitor, i.e. it becomes your automatic last day when you accept the outside offer. Some folks give their bosses a heads up that they're planning to accept something outside that would put them in such a circumstance, but
not everyone.
Case in point, I know of one person who despised his current leadership so much that he strolled over at the end of the day, told his boss he'd accepted a gig with a competitor and that being the end of his last day, turned and walked out.
Now, would I do that to my current leadership, no. Would I do that to at least one inhuman piece of trash I worked for in an earlier part of my career at the same company?
Absolutely.
Loyalty is a two way street. When a leader or a company shows their employees no loyalty, they should expect none in return. Yes there are times when you should be the better person and do right by the company, but there are also times when turning the other cheek only gets you slapped harder. The folks that talk about getting fired after they try to put in two weeks notice would be an example of the latter.
My only other parting contribution is that some bridges are worth burning, even if only to make sure you never go back across them yourself.