I think altruism towards humans is the greatest good. Have I implied otherwise?

As regards the first half, I understand your point. But I think you're deceiving yourself. Whether you use "should" or not, you're endorsing a point of view of how you think things "should" be, whether you'd push them on others or not. "Should" implies a goal, I don't believe it implies any inherent power or dominance or coercion. It's simply an opinion. Just because some assholes -- like the Church -- say you "should" do something doesn't mean they have a monopoly on the definition of should.