Why is altruism good?

I define altruism as a creed which holds that helping others who are suffering, regardless of the conditions of their situations, as a moral imperative. I've yet to see a rational argument supporting this. Wouldn't an altruistic society survive by taking the strongest, intelligent, most productive members of that society and sacrificing them for the benefit of the weak, stupid, and unproductive? Wouldn't it basically punish virtue by expecting anyone who excels to sacrifice themselves and reward vice by giving the unearned to those who are the least productive? It seems as if, ultimately, altruism encourages self-sacrifice and self-negation. It turns the virtuous men into the slaves of the weak. How on earth is THAT virtuous?