Compare these scenarios:
You did not take up arms against the invaders, but as it turned out, you were captured. Forced labor, illness, and death await you.
You did take up arms against the invaders, but as it turned out, you lost and were captured. Forced labor, illness, and death await you.
The result is the same. But in the first case you did not give in to the temptation to answer evil with evil, saving your God-given soul. And, in the attempt to restore peace through goodness, this grace was multiplied. In the second case, the soul gave up and put itself in mortal danger of hell.
But this is not the worst scenario.
The worst is when you take up arms and win. You defended yourself, your loved ones, your territory by murder. But, all around you, the anger and hatred was multiplied. Now, not only is your own soul in mortal danger of hell, but also those of the people around you.