From both Canon and Legends we know that the Sith are egocentric and selfish, lusting for power for themselves. The last line of the Sith Code says:
Through victory, my chains are broken. The Force shall free me.
Emphasis on "me" and "my", no consideration for others. Things like friendship, loyalty, mercy, and compassion are considered as weakness by the Sith.
Therefore, any true Sith would simply not believe in a cause higher then himself. A true Sith would not sacrifice himself for anything, especially not for some abstract ideal like the victory of the Dark Side, the future of the Sith Order, the defeat of the Jedi Order after his death, etc.
Yet, according to the Rule Of Two, any Sith Master is obliged to spend a large part of his life (approximately at least 10 years) in training his apprentice, his potential killer. The apprentice would naturally want to kill the master, but if the master kills the apprentice then all that time spent training him would have been wasted. And all that the Sith Master gains from his effort is the possibility that some future Sith would defeat the Jedi and rule the galaxy.
Personally, I find this belief in a "higher cause" quite contrary to Sith nature. Wouldn't it be more logical for each prospective Sith to first work on his own immortality (or at least prolonging life), so that he has more time to gain more power? And if he cannot do that, at least conquer some remote world and rule there for the rest of his days (actually many Sith did that)? Any other Force sensitive would be seen as a potential threat, especially those with higher Force potential. It would be prudent for our Sith to eliminate them if he can (reducing competition) and not to train them so they could kill him.