I was watching Ken Burn's documentary "The Vietnam War" a North Vietnamese soldier said "War awakens a savagery in people. Not even animals are that savage"

Matt Harrison, a West Point graduate, who was a commander in the war said,
"The veneer of civilization is very thin. It doesn't take long for the veneer to erode and we are capable of doing inhuman things."

I'm not a psychologist or sociologist, but at the age of 70 one of my big questions in life, is what happens

I raised sheep when I was a kid and awoke early one morning with 3 or 4 neighbors dogs chasing the sheep taking them down and ripping skin off their legs. I tried to patch them back together the best I could, but one died.

These were friendly neighborhood dogs, which by themselvs wouldn't hurt another animal. Well maybe a mouse.
But, together their pack hunting instinct from their ancestors, wolves, took over.
There is certainly a "tribe" instinct in humans that can account for things like racism in humans.

I did a quick search.

Are humans-the cruelest species on Earth | Quora.com

Some say yes, because we are the only species to kill other than for survival.

"It seems that self-awareness and empathy bring with them the capacity, and altogether too often the desire, to inflict distress and pain on others simply for the sake of doing so. Cruelty has been documented in all self-aware species, from elephants to dolphins, great apes and more."

Humans, are the only species who have a special emotion. It’s called revenge.

It is reported that ancient Aztecs used to perform human sacrifice where the beating heart was extracted from the unfortunate war prisoners and dedicated to the Sun God.

The Japanese soldiers in the rape of Nanking, for example, competed to see who could be the most horrific, acting similarly to Nazi soldiers competing to see how pointless and emotionally degrading their abuse of concentration camp prisoners could be.

Why do we turn into an a-moral person in war? I may have read this somewhere and need to try and find some scientific paper on it.
Humans have a more evolved Neocortex (outer brain which does things like logic, will, ...)
Our Limbic system - Emotions (fear, anger, ...) can be overriden by the neocortex.
In stressful situations the neocortex may not be able to override the Limbic system. This may have to do with some survival instinct or probably something more complex that psychologists can explain.

See:
Neuroanatomy.
Turn Down Your Brain’s Worry Center | Psychology Today

See:
Evolution of Morality - Yale University | Coursera