The reconciliation makes Laertes' treachery (using a poisoned sword) all the more terrible. Hamlet believes that Laertes has accepted his apology and explanation, and has forgiven him for killing Polonius. Hamlet says that he is not responsible for Polonius' death:
Hamlet does it not, Hamlet denies it.
Who does it then? His madness.
And Laertes says that he will consult an expert about the honor of accepting the apology, but until he hears from the expert,
I do receive your offer'd love like love,
And will not wrong it.
In reality, though, Laertes knows that he is about to duel Hamlet using a poisoned sword King Claudius gave him.
Laertes is generally an honorable man, so his willingness to lie and use poison shows that killing his father, Polonius, was unforgivable, and Laertes will do anything to avenge him. This is in direct contrast to Hamlet, who stops to think about the morality of avenging his father's murder, and does not attack Claudius until the final scene.