but he's a Sword through and through, not a Cup.

Which is interesting because of his name - loose (Lucas-style) translation of sword-belt or something.

I have this vague idea of Obi-Wan as having such a tolerance for his own horrible experience in life that he thinks others should be able to bear suffering like that, too. Because that's part of life and that's the right thing to do. Doing his duty and 'the right thing' is so important to him; he doesn't show much mercy to those that fail in that.

I wonder (given his 'certain point of view' ideas) if perhaps he saw killing Vader as a compassionate measure?