Teaching about Hell is not cruel, for do we not teach about the dangers that exsits on earth and what type of life or acts could make us fall into such danger?; is this done out of cruelty or is this done out of love and care?; it's obviously the latter, for we warn people in order that they may beware and stay on the straight and narrow and thus not fall into danger.

It is only right that there should be punishment for the evildoers in the hereafter and that there should be reward for the good people, so God has prewarned us about the hell-fire, and pointed out the path that leads to it [via Prophets and heavenly revelations], and it is our duty to pass on this message to all mankind.

Peace.