Eh? When was that ever established? In "Journey to Babel," Spock says explicitly that his father is perfectly capable of killing, coldly and logically, if necessary. And that's the episode that introduced Sarek. I don't recall him raising any objections to the military nature of Starfleet at all. The issue just seemed to be that Spock had defied his father's wishes regarding his career. Vulcans can be quite ruthless when they need to be. Look at T'Pring or Valeris . . . or even Spock on occasion. Don't forget, he spent most of the second pilot urging Kirk to kill Gary Mitchell before it was too late . . . .