It does seem clear that Kirk murdered Kruge's boarding party without any remorse or "sporting chance" or other such nonsense. And why he chose to perform this necessary action via self-destruct rather than via beaming-into-oblivion seems clear to me as well...
The ship was going to be lost in any case: killing the boarding party would prompt Kruge to fire on the unprotected Starfleet ship and easily and completely destroy it. So our heroes had to get out of the doomed vessel in any case; the self-destruct ruse allowed for that, but staying behind and fine-tuning a transporter to accomplish the killings would not have.
Timo Saloniemi
The ship was going to be lost in any case: killing the boarding party would prompt Kruge to fire on the unprotected Starfleet ship and easily and completely destroy it. So our heroes had to get out of the doomed vessel in any case; the self-destruct ruse allowed for that, but staying behind and fine-tuning a transporter to accomplish the killings would not have.
Timo Saloniemi