The Capellans nailed in that rock slide caused by that sonic doohickey he and Kirk rigged up. Those were boulders. We're never shown any of them dead, but it sure looked like some of them had to be killed.
And later in the episode, Spock does clearly shoot one Capellan with a bow and arrow.
Spock assists Kirk in rigging the grenade launcher in "Arena," though we don't know how much damage it did.
In "Wolf in the Fold," Spock beams the Redjac-possessed Hengist into space on wide dispersal. Hengist was already technically dead at that point, only animated by Redjac, but it was assumed that dispersing Redjac would eventually kill it (although a couple of different comics sequels have argued otherwise).
In "Return to Tomorrow," Spock was sharing consciousness with Chapel when she injected Spock-Henoch with what Henoch believed was poison, resulting in Henoch's flight from Spock's body and thus his death. It's unclear whether Spock was fully in control of her actions at the time, though, and one can split hairs about whether tricking someone into killing himself is equivalent to killing them oneself (although I'd say it is, from an ethical standpoint).
Similarly, Spock is the one whose actions drive the Zetarians from Mira Romaine's body and kill them in "The Lights of Zetar."
And of course it was Spock who rigged the torpedo that destroyed the Klingon ship in TUC. (Infuriatingly, he did so with McCoy's help, despite McCoy's insistence in "The Empath" that he would not take life under any circumstances, and despite the fact that it made no damn sense for a medical doctor to modify a weapon.)
There were times when Spock advised others to use deadly force -- suggesting it as a way to deal with Mitchell in the second pilot, telling Yeoman Tamura to use lethal force if necessary in "A Taste of Armageddon," urging Kirk to kill the Horta when it threatened him in "The Devil in the Dark."
(Although I've never believed that Spock was actually trying to convince Kirk to Gary Mitchell. Rather, he was trying to convince Kirk to strand
Mitchell by pointing out that killing him was the only other option.)