Anger has a biochemical effect on the body and the brain. Fear is forgotten. Adrenaline courses through the muscles. The natural state for fighting is aroused.
A simmering anger also is effective in motivating troops over the longer term. In this way the atrocities of the enemy may be held up and amplified such that they create a longer-term hatred that drive the army on and on until victory is gained.
The desire for justice is a result of a feeling of betrayal or that sacred values have been transgressed. When victory is the means of restoration, then soldiers will single-mindedly seek the defeat of their bitter enemies.