Hamlet's father was unjustly MURDERED by Claudius while he slept in the garden. Claudius crept up, and killed Hamlet Sr. by pouring a deadly poison into the "porches" of his ear. This is the most unjust act of the play and it is why Claudius is the antagonist.
Laertes's death can be seen as "unjust" because he was led untruthfully by Claudius for such a long time. Laertes was really only trying to avenge his father's death and the cruel treatment of his sister. There isn't anything really unjust in that, if we follow an "eye for an eye" philosophy.