Tau like to preach about the Greater Good (the greater good) but they're no valiant champions of morality. Least shitty by far, though.

Orks are questionable because they do cruel things without purpose except the cruelty. It's arguable that that's just how they're genetically programmed what with being artificially created and all that, like how I would argue the tyanids are the only "good" faction by virtue of being true neutral and at many stages more or less animals.

All the other factions do evil shit to whoever they feel is standing in their way, including their own people.