Are the Eldar actually good guys?

[John Bower]

What kind of question is that? There are NO good guys in the Warhammer 40k universe. At least none that are alive and numerous enough to constitute a faction. However, the Eldar are still the best among the bad guys.

Actually the Tyranids are... No concept of 'good' or 'evil' just eternally starving hungry. Eldar are evil like anyone else. But the Tyranids don't do anything out of malice or a sense of misplaced snobbish pride. So actually they are the best of a bad bunch.

[Lord-Boofhead]

force-evolving the Tau to their current technological high-point to act as a buffer against the horrors out there, forcing them to die instead of precious, precious Eldar?

I have never seen anything that even vaguely suggests that? Where does that come from?

[YorkNecromancer]

It's never been outright stated to the best of my knowledge, but it is heavily impled that the Eldar modified some Tau to create the Ethereals (who the Tau codex says basically came out of nowhere), which then led to the formation of the Tau Empire, and the sudden shift of the species from agrarian plains-dwellers to the super-advanced technology users we know them as today.