One of the most important teachings Buddha forwarded in his life was to be compassionate and harm and kill no form of life. Buddha had once remarked that one should neither kill nor cause to kill, and in modern life, this is a lesson to learn and remember. How?

Well, although Buddha referred greatly to the physical violence at his time, today, this harm can be physical, mental, or even emotional. The words we say, the acts we do, the way we treat the fellow man, the way we treat a creature as little as a fly, it should all be done with compassion.
One should not think about hurting even a small insect, let alone a fellow man.