It is not idealist, there are plenty of studies to show that people who give charitably are happier and happier people give, creating a happiness loop.

If given the choice of being robbed or giving to something that you care about, which would you choose? Which do you believe would bring you more happiness?

No it's not.  There's what's right and what's wrong.   When slavery was around there was a small amount of people around who knew it was wrong.  The rest of society either didn't know because they hadn't thought about it or used the excuse that ii had been around a long time, or that the majority approved or that it was necessary or some combination of those.  Unfortunately, that's all it was, an excuse.  Same is true for today for taxation.  Either people haven't thought about it or they're making some excuse for it because they either benefit or they don't have the imagination to think of life without it.   But it's just an excuse, because there are people around pointing out the immorality of it just like there was back in the heyday of slavery.

And the same is true of human sacrifice or any number of accepted immoral traditions throughout history.   

Man started in this world not knowing much and built all these traditions upon bad information.  We now live in a more enlightened age.  It's time that we were brave enough to recognise what is immoral rather than kowtowing to something just because the majority is currently going along with it.  But history shows that only a minority stand up for what's right whereas the majority just go along with whatever they are told.