[member="Wolf"]
I've always written my Sith as ultimately 'nice', primarily because the role of the Sith is not to be self-serving, egotistical and sadistic - that, sadly enough, is a byproduct of their philosophy, which states that strength is that which should be respected above all. This derives from both evolutionary and political understanding: the 'strong' ultimately survive in conditions where life is unable to thrive, either through overt force or having the ability to adapt to changing circumstances (much as Darwin suggested). That latter part is why Sith training ultimately focuses upon enduring trials and challenges - those capable of being effective Sith ultimately survive them (intact or otherwise), while weaker, less-deserving candidates fail (and usually die).
This moves onto political views: essentially, the Sith believe that the weak should rule over the strong, simply because they have the power and insight (both from experience and the Force) to maintain peaceful order. In essence, the strong rule and any opposition to this, or from those who might seek to subvert the system (smugglers, pirates etc) are destroyed as the threat to the system. This cannot work in a Democracy unless there is a strong military power base (which the Republic traditionally did not maintain) - in essence, anything that acts outside the defined parameters of the system are destroyed through massive force. This extends as much to personal power as it does to political and military power. An individual Sith must be strong in order to rule - they must be able to use their wit, cunning, intelligence and brute force capabilities to maintain their status. After all, a weak person can be walked over by those who choose to ignore them. That can't be said for a Sith: to oppose one is to sign your own death warrant.
Sadly, this understanding tends to be corrupted by the Sith themselves: as each Sith trains, they are forced to overcome challenges and become stronger through dealing with adversity. As they do so, they ultimately see themselves as part of the Elite, whether in truth or through aspiration. As such, everyone beneath them is weak, and Sith ideology often (incorrectly) suggests that what is weak should be destroyed. In truth, the Sith suggest that the weaker need to be controlled and led - thus leading to that sense of Sith superiority/domination. Since other Sith can't be controlled, if they are weaker, they are opposition rather than subordinates, which is how most arguments and battles among the Sith start: one sees themselves as superior and seeks to assert dominance, the other resists, and so they fight until one is dead. This is why the One Sith doctrine operates here: all Sith are required to understand that they serve the Emperor's will, and thus must focus on a higher purpose.
Thus, what we come to is the doctrine of useful resource: if it can be used or put to functional purpose, it will be. Where some Sith would slaughter an entire army to make a point, others would simply kill the one or two at the top of the hierarchy in order to end resistance, then turn that force into one that will fight for the Empire, thus repurposing them. The simple key is that you don't waste anything that can be useful: thus, being an ethical Sith is a question of pragmatism. You *could* kill entire planetary populations to assert your power, or you can capture them, put them to use as a means of strengthening your power base and widening the scope of your authority.
Ethical Sith are ultimately those who work purely to benefit the Empire: that's the role of each Sith, according to the Emperor's vision. You don't randomly kill people, because they could be put to the service of the Empire in turn - everyone has a function to fulfil. Thus, the sadistic acts of killing go away - the Sith serves to strengthen the Empire, and achieves this through strengthening themselves and also from using their strength as an extension of that belonging to the Empire as a whole. We're not here to butcher all who bother us.
