Hamlet wanted to punish evil, but he caused the death of loved ones. Why?

Because that was the true intention of the author. What we have for centuries perceived as a just retribution to the enemies of Hamlet, in fact, looked different. In Shakespeare’s time, ghosts were nothing more than the incarnation of the Devil. Thus, Hamlet followed the advice of Satan. The ways of fighting evil are very thorny and do not always lead to victory.