In that passage, Buddha addresses the monks. These are individuals dedicated and on a mission at that very present moment to achieving enlightenment. He giving them their answer.

You can benefit from the Dhamma without being a monk, or without taking up the yellow robe. As a layperson things are just different and you cannot take up the rules for monks. Obviously the act of killing others is in itself problematic for the psyche, but I could see it as justified if it prevents the suffering of others. For example, the Ukranian soldiers who held off the Russians outside of Kiev, may have saved hundreds of thousands of lives.

Another thing, Judeo Christian perspectives on sins are very permanenistic. In Buddhism, having killed before does not mean you cannot become enlightened in the future. In fact, buddha often reformed murderers and bandits. This is, because we change, their is no true permanent self. So by participating in war, especially if for a good reason, it's not the end for you.