If you think about it, only one faction in the whole of
40K is even close to "Good" and that would be the

Tau Empire.

And maybe, just maybe, the Eldar.
Think about it-
Followers of the Emperor (Humans) Are sole content on destroying anything alien, heretic or machine; What I'm saying is, essentially, the Emperor is right-winged.
Next are the Necrons who, quite obviously, are evil. Set out to destroy all life, anyone who calls them good needs help. And if you ask me, they are most likely to come out on top.
The Orks live for war and killing, and are bred for the fight.
Tyranids are, by instinct, ferocious. Some may argue they are not evil, but... no. They have supposedly consumed galaxies, and the universe is better off without them.
Chaos is hell-bent on blowing up, burning, ripping and shooting everything. The living embodiment of evil, down to the last stinking Nurgling.
Dark Eldar are the pirates of space, with a sprinkle of witchery and a slight hint of chaos. You simply can't call them friendly.
As for the Eldar, they fight to save their dying race, only to save the craftworld.
And the Tau, they only fight to bring peace. But in the 41st millennium, guns speak louder than words.
What do you people think?