You don't have any more "right" to sit on excess than I have a "right" to steal it.
Instead of using an amorphous concept like a right I'll put this in terms of when the use of force is justified. I don't agree that your example is a case where the use of force is justified.
I do agree that it's wrong for the individual with excess not to be charitable or to put that excess to good use, but that's not the same as saying it's okay to use force to take that excess and be 'charitable' or do good with it.
Greatest good for the greatest number, this is why we don't still live in fucking caves
Yes, absolutely. And this is best achieved when people recognize that it's not right for them to just take what they want from other people, even when they think they need it more than the people that have it. This rule of law is indeed an important pillar of civilization.