At that time those were not his lives to take. Add on the fact that he went rogue with a Federation starship in which he was responsible for countless lives.
None of this differs from what Kirk did in ST3:TSfS. He stole a starship and murdered a bunch of Klingons. While the Klingons apparently declared war over it (or at least said "there shall be no peace as long as Kirk lives"), nobody paid much attention, just like nobody really noticed when the Cardassians decided to be at war with the Federation at the start of "The Wounded". And while Kirk's superiors didn't exactly like him going rogue, a year later Kirk was a celebrity and a few years on a leading envoy to the Klingons.
Starfleet doesn't much care if its field commanders kill people. Space is dangerous enough, with grave dangers of misunderstanding either way when encounters with the unknown take place. Sometimes skippers err on the side of caution and get themselves and UFP citizens killed. Sometimes they err the other way and get alien scum killed. Obviously, Starfleet would vastly prefer the latter.
This reflects oddly on the sentiment that starship crews would be expendable in upholding the Prime Directive, with altruist suicide preferable to selfish survival, as suggested in "Omega Glory". But said TOS episode is basically the only place where anybody from Starfleet ponders on the virtues of suicide as a military maneuver. Kirk flat out refused to consider it even in cases where he could take down key UFP enemies with him. Picard spoke of the importance of upholding Starfleet principles but never when self-survival was at stake. Both were soldiers by profession, and both swatted starships occupied by hundreds of sentients out of existence whenever necessary for UFP safety - without formal declaration of war or anything like that ("Day of the Dove", "Best of Both Worlds" etc).
Timo Saloniemi