Spock believes that Kirk is about to be killed. Kirk is the one who decides to wait until it attacks. What we really have is the repeated example of how Spock and McCoy offer advice, but Kirk makes the decisions.
Again, very true,.. Spock represents the Logical, McCoy the Emotional, and Kirk the enacting body and final arbitrator of his internal conflicts which are personified externally by the Spock and McCoy characters in conflict.
However, in the Horta case,.. as it relates to Spock violating his Ethical Standards,... Spock cries out for Kirk to 'KILL IT', which as I stated before, violates the whole individual sacrifice for the good of the many, all life scared, Reason over Force, all those moral/ethical themes STAR TREK put forth, but then let have it with full phasers - when it got in the way of FEDERATION BUSINESS, or the STARS BEING HEROES.