Can I tell someone I'd watch their luggage and then abandon it?
Are you serious? What do you hope to achieve by leaving people's luggage unattended?
I like to watch their faces when it happens.
If you say you will watch someone's luggage, then you are responsible for it. Walking away from people's luggage is therefore irresponsible. Follow through on your duty and watch the bags.
I don't care about the consequences so I will keep doing it.
You need to stop. Eventually you might get in trouble and have a hard time not being able to get on a flight. It is also mean to offer to help someone and then not do it.