The T'au Empire had done plenty of bad things too. For instance, they coerced and bribed other races into joining their empire. Those who refused were immediately labeled enemies of the Greater Good and crushed.
If they truly could not get a positive response, then no one could blame them for using railguns to blast open the gates of a nation.
But at least they still put in a little effort to communicate with aliens. The Imperium of Man could not even be bothered to talk anymore; they simply started killing.
Still, the Imperium of Man could not really be blamed for that. During the Great Crusade, The Emperor had embraced a policy of tolerance rather than dedicating himself to interstellar genocide.
Unfortunately, the aliens the Imperium of Man had faced back then were so utterly deranged that reconciliation was impossible. They deserved only to be ground to ash beneath the treads of war.
The T'au could only be said to have arrived at the right time. The Imperium had already killed off most of the most dangerous aliens, and the old alien remnants trembled at the sight of the Imperium of Man's treads.
The aliens of today were either newly emerged species or threats too minor to have been put on the Imperium of Man's urgent purge list during the Great Crusade.
If the T'au Empire had been born in the Ghoul Stars, the Imperium of Man would not even have needed to act. The freaks and monsters of the Ghoul Stars would have taught the T'au Empire a proper lesson.
Seeing themselves as destined by the cosmos, they regarded themselves as superior beings and all other races as savages.
But that was not much of an issue, really. After all, everyone in the Warhammer universe thought that way.