At the start of the play Laertes is a concerned brother and a dutiful son. He understands his place in court, as the son of Polonius, and fulfills the ritual of asking the king's permission to return to school. He expresses a serious concern to Ophelia in regards to her relationship with Hamlet. He makes a very valid point that no matter how Hamlet feels about Ophelia, in the end, princes don't get to make all their own choices, especially in regards to a wife. Prince Hamlet may not "carve for himself, for on his choice rests the health of the whole nation." He warns Ophelia to maintain her virtue, but he doesn't condescend to her (too much) and doesn't discount the feelings Hamlet and Ophelia have for each other. As regards his father, he patiently listens to the laundry list of fatherly advice before heading back to school.
We don't see or hear from him again until the middle of Act 4, but when we do, we see a very different man. He is furious over the death of his father and initially blames Claudius for not reigning in Hamlet in his craziness. He is ready to punish Claudius. After he is calmed, he then expresses his anger and outrage with Hamlet, even saying that he would "kill him in a church!" Claudius uses this emotion to his advantage and manipulates him into the plot to lure Hamlet to a fencing match where they can use poisoned swords and wine to cause the death of Hamlet. He is especially driven to this act when he sees how Ophelia has gone crazy, and later when he learns that she has let herself drown.
While he begins the sword fight with a lot of verve, he is pretty quickly swayed to forgive Hamlet his past actions. As the sword fight continues, he strikes at Hamlet with the poisoned sword in a moment of frustration, but once he knows he too is struck with poison he quickly reveals the whole plot so that Hamlet can punish the man who ultimately is responsible for all of this -- Claudius. Leartes seems to have regained his conscience; at the very least he didn't want to die with that secret on his soul.
Leartes transforms from a gentle young man to a man of extreme passion and aggressive action. Unfortunately those actions were not well thought out, and he is destroyed by those actions