This particular moment in Star Trek is actually quite important. A lot of people don’t realise that understanding something is not the same as approving of something. This particular episode (A Taste of Armageddon) had a civilization where war was fought on computers instead of on the battlefield and instead of people dying in combat they would send the calculated amount of “casualities” into a camp to die. Kirk is outraged completely by this and rightly should be, but Spock is not so overtly disapproving. He understands why they might think their solution is better for their civilization and takes the time to think about why they are doing it. Even though he can understand why, he still believes it is wrong for them to be doing it.
There is a separation between understanding something and approving of something that a lot of people seem to miss.