Warhammer has no good guys. All are bad, but some are mod bad than others.
Chaos: Name says it all. Endless slaughter, killing, chaos.
Space Marines: Servants of a God-Emperor of Man, love to kill anything not human (aliens, mutants, heretics (anyone who doesn't worship the emperor) and let's not forget the Exterminatuses they do.)
Tau: Complete and utter Communists led by the self-righteous, all-knowing, and hypocritical Ethereals. Need I say more?
Sisters of Battle: Take the Space Marines, now add women's hormones to the mix. Hell hath no fury like an army of virgin women scorned. If someone would just screw them and remove that stick from their collective a$$es.......
Orks: Ever play Warcraft? Now, imagine them even more bloodthirsty, and having no sense of honor or loyalty. Evil enough to make Sauron's orcs run away in terror.
Eldar: Sharing that same self-righteous with the Tau Ethereals, the Eldar are the supreme ****s of the galaxy. Really. They are an entire species of jerks. Humans at least admit we are jerks. They deny it. HYPOCRITES!!!
Dark Eldar: Take the forces of Chaos. Now, add advanced Eldar technology to the mix. And if possible at all, make them even more twisted and evil. How are they more evil? Simple. They don't like chaos. Their evil is coldly and precisely calculated to as evil as inhumanly possible. They're the Nazis of the 41st Millennium.
Necrons: Dark Eldar want to at least keep everyone else alive as slaves. Necrons just want to kill everyone else for the heck of it. Plus they're even more cold and calculating, especially because they're undead machines (how can you be both undead, and a machine? Its like the creators sat down and said Zombies are evil and machines can be evil. The only thing more evil would be zombie machines. Yeahhhh!!!! Evil, undead robots would be sooo evil!!!!).
Imperial Guard: Imagine the old Soviet Army. Now add religious rule to it. So you have commissars, lots of summary executions, strange disappearances, and brutal mistreatment of the soldiers. Now add religious preaching to it. Estimated suicide rate of your average Guardsman? 0% according to official, completely trustworthy sources. Number of worlds completely depopulated because average Guardsman has a life expectancy shorter than a fruit fly? How many licks does it take to get to the center of a Tootsie Roll Pop? Who is John Galt? The galaxy may never know.