Well, here's my reasoning:
>Tau do everything for the Greater Good and seem to be the least belligerent faction overall, even being willing to engage in diplomacy. They're "good" by virtue of having no plans for genocide.
>Eldar are huge dicks who exploit everyone else for the benefit of the Eldar race, but like the Tau, are ok with the survival of other races and don't really make any more trouble than they have to.
>Imperium are Catholic Space Nazis. They want to kill everyone else because the horrors of the galaxy have given them collective shell shock and they've learned millennia ago that nobody can be trusted. They're evil because they're paranoid and fearful.
>Necrons are arguably less evil than the Imperium in the new lore, but fall lower due to roughly half of them being mindless death machines. The intelligent ones mostly just want to rebuild some vestige of their long-dead empire and take back stolen artifacts. Have cooperated with Space Marines in the past.
>Tyranids have caused widespread destruction and misery for every faction except the ones that are into that sort of thing. They aren't really evil in the sense of active malice, but they've probably killed more people than all the other factions combined and their weapons tend to be unbelievably cruel.
>Orks, as noted above, are the backwater savages of WH40k. They just want to fight everyone for their own amusement, and they don't really seem to understand the concept that other races value their lives.
>Chaos is super evil. Read up on the Daemonculaba
if you don't believe me.
>Dark Eldar are even worse, their entire culture revolves around rape and torture and their spaceships are literally fueled by suffering.