I figured that was probably what you had in mind. But "most good" isn't really the right term .
http://en.wikipedia.org/wiki/Utilitarianism is a theory from the ethics branch of philosophy (the one that deals with morals, basically).
The humans are nationalists to the point of fascism, who will kill, torture, or worse anyone who does anything that could be construed as contrary to the good of the Empire. Wiping out whole planets isn't even the most they'll do, and they don't always have good reason for it.
The Eldar will allow any number of aliens to die if it is to their slightest benefit, and will sacrifice anything if the future gains are greater.
For the Tyrannids and Necrons, there isn't even any observable sense of self, so they fall into the "for the greater good!" category by default.
At least the Tau empire tries to make life nice for people (that is, after all, its stated goal), it just expects all its people to about as altruistic as it is possible to be.
As to the Tau empire and freedom. it allows the humans within some autonomy, and to worship the Emperor if they want to. They're more like the Roman empire than the British empire: they absorb and influence, rather than imperialistic colonisation.