Given all that, was this a selfish and immoral decision?
No, you took a decision which is good for you, nothing immoral about making the decision itself to move out.
Shouldn't I've hurt my boss by not breaking promise and not quitting the job?
Yes, Absolutely. Let's take it in two parts:
- "breaking promise": Yes, Absolutely you should not have done that.
- "quitting": Back to answer 1: choice is yours.
Moral of the story is: You should never burn a bridge. You are the authority to make your decisions - but not through a broken promise which can be perceived as a lie. You should not promise things which you cannot keep. Learn a lesson, and move on.
Next time, when you need time to think to take a decision and make your mind - just say so. Take time and get back with a definite answer. People appreciate a well thought-off decision which may take some time, and eventually despise a prompt promise which is broken soon after.
Note: The way your boss acted (or reacted) is also not professional and uncalled for (even when you seem to brake your verbal commitment), but that's not in your control, and should not affect your decision to move on. Every now and then, people face (which they should not) these sort of "threats" or personal grudge-based comments when they announce their separation from the company. Not common, but neither unheard of.