Isn't it inhumane and unforgiving to sentence prisoners to life in jail?

It is not inhumane to sentence a murderer or a child rapist or a habitual criminal to a very long sentence. Your argument forgets one very crucial piece. The victim, followed by the victim's family. It wouldn't be fair in the least to them. And lastly, it wouldn't be fair to society as a whole to learn that a murderer or a rapist lives among them only because said criminal met the minimum diluted requirements of a sentence.