What's Lucas talking about here?

There's morally right, legally right, and the practical side of things.

In legal terms, Anakin was right to save Palpy.
In moral terms, who the heck knows, given Palpy's true nature?
In practical terms, offing the guy woulda spared everybody a boatload of trouble.

So I guess you have to ask yourself: What kind of 'right' are we talking about here?

 

let's face it, the only reason anakin chose to save palps and sacrifice mace was because he thought he stood to gain something from sidious. otherwise, killing the sith master, who was hardly unarmed, would have been no different than killing the apprentice (maul,dooku). in fact, it's arguably the right thing to do.