Captain Tracy wrote:
. . .Spocks plea to Kirk is to Kill the Horta - the last of its kind, and mother to hundreds of thousands of offspring,... just to save his personal friend. [Emphasis added]
But none of this is known when Spock recommends that course of action. All they know is, it is a dangerous creature that has killed several people. The mind meld giving Spock knowledge of the creature's motives hasn't happened yet.
Too true,.. however, we have all heard the "all life is sacred" speeches prior to this episode; and Spock's descsion and actions were in just as much violation to his rhetoric in past episodes as in this episode,.. the 'Horta' is just the most overt I could think of off the top of me head,
Also, Spock does not adopt this philosophy only for the Horta, and only suddenly after the Mind-Meld,.. it is long established and re-inforced through the series run in defining his characters belief and value system,.. the facts concerning the Horta race mortality only serves to increase the drama of the conflict,.. it does not suddenly define Spock's intrinsic long-held standards, which are (supposedly) applied to all things great and small.