Varsovian:
Hm. Tell me more. :) How do you make a difference, when the Empire you serve is so awful? Or when your goal (as in DH) is to kill everyone who thinks differently?
And I really can't see how do you make a difference in BC :)
It is all from the human perspective.
Lets compare notes.
You have the Eldar, who will do anything and sacrifice anyone else to save even one of their species. They are dying but they will not go down without a fight. That means that will make sure billions of humans die so that a couple hundred of them might live. They also have cousins who are Dark Eldar. They are exactly the same except for one differance. While the Eldar try to fight the Chaos, Dark Eldar serve them. Dark Eldar harvest souls to offer to Slaanesh so they can live for another day.
You have the Orks. Who are genetically engineered warriors. They will stop at nothing to fight. They will fight themselves, Imperium or anyone else. They will travel across the starts to that they can get a good fight. Problem, their reproductive system are spores. They constantly release spores especially when they bleed. These spores becomes mushrooms, these mushrooms becomes Orks. And once enough Orks are born they can make weapons and armies etc. They will kill anything and everything because its fun.
Next you have Necrons. Who are basicly robots in a sense. Their objective is to erradicate all life from the galaxy so they can win their war against the Chaos Gods.
Than you have Tyranids. Starship Troopers + Alien equals these guys. They are known as the "Great Devourer" because they consume any organic material they come across. Vegetation, animals, people, they even comsume the air. Not a pleasent concept and not pleasent to have around. They are practically unstoppable, and they will one day eat everything, if the Orks don't beat them.
Than you have Chaos. You have four gods. Khorne who is the god of slaughter, blood, combat also the god of honor and bravery. Nurgle is the god of decay, sicness, pestillence also the god of acceptance and medicine. Slaanesh is the god os lust, perversion and also the god of love. Tzeentch is the god of change and mutation also basicly the god of hope. They gain power from any and all emotions and twist them around to their own gain. Also because of this, in the end things tend to die.
Finally you have the Tau. They have great economy, infrastructure and a stable government. They comperatively treat everyone nicely, they educate them, take care of their problems etc. Basicly they are space communists. For the greater good and all that. Problem is, there are quite evident signs of mindcontrol or brain washing involved.
Compared to them you have the Imperium. Its machinations are influenced by these aforementioned powers and designed to fight them or their influence. Desperate though it may be they are at least trying their best.
There are also a whole lot more aliens involved but their are either too few in number or very limited to a certain place that they get no mention here; but they are no different, they will kill or mess things up.
Basicly, if you somehow stop the influence of these enemies, or halt their advance, it is considered a good thing. Even though you had to erradicate an entire planet to do so. Things are messed up.
As for your BC question. As I stated above, Chaos Gods might sound and seem like evil entities but their domains also cover good things. You can be the most brave and honorable servant of Khorne, or the most dutiful and life saving servant of Nurgle, or someone who gives hope to untold billions serving Tzeentch. You don't have to be a killing machine. It's just that, you actions kind of always go around and become bad. You give hope to people, they start to believe it, and billions die and as a reward you end up as an unrecognizeable but ultimately powerful blob. You stem the tide against dissease and sickness, save millions, but the methods you use cause a new and much improved dissease that kills billions and as a reward you end up as a bloated living dissease bomb. You can be the most honorable and brave fighter of all, but as you win more and more fights you will become more and more affected by rage until one day you will kill anything on sight.
Differance being, the way you get there. You can choose not to give in to temptation, try your best to not fall trap to the easy ways and you can avoid corruption and damnation. It is far easier to give into temptation and corruption though as in Black Crusade how influenced you are by the chaos gods determines extra effects. The more effected you are the powerful they become.
Edit:
Addendum:
Yes, you read that right. The god of hope is an evil manipulative thing. The rest of it becomes kind of moot after that.
This message was last edited by the user at 07:10, Tue 19 Aug 2014.