Coming twelve centuries after the life of Jesus on Earth, and reflecting much theological and pious development, the words of the Doctor Angelicus cannot be used as a source for Jesus himself. Nevertheless they are a fine and noble addition to the tradition.

However the thought is very very easy to abuse. Too much of "jus ad bellum" can be simplified on the grounds that "we're aiming at peace, aren't we." And then once the "jus ad bellum" is taken care of, "jus in bello" has rarely been so scrupulous as to condemn more than a few "bad apples." 

And in connexion with "killing is always an evil," even when it is excusable, as perhaps in a truly "just" war, we should note that no true Christian "just warrior" will ever celebrate his having killed, or rejoice in it, or boast of it. Nor will that warrior's society. Killing is always an evil, always a source of sorrow, always to be regretted.