What does it mean to be a human?

What does it mean to be a human?

As I read through the chapters of Honda Katsuichi’s book "The
Nanjing Massacre: A Japanese Journalist Confronts Japan’s National
Shame", one sergeant confessed that because a crying baby disturbed him while he was raping the baby's mother, he threw the baby into the boiling water. Was that Sergeant a Human?

Before telling something is human and something is not, we first need to define What does it really mean to be a human. Does having a human-like appearance will make me a human? Does speaking a Human Language will make me a human? If so, then the Sergeant who threw a baby in the boiling water is a human, the abusers of "Abu Ghraib Torture" are also a human, the doctors who performed "Nazi Human Experiment" are human too.