Although no act is named, it's in Act 4, scene 5 that Laertes bursts in on Claudius after having heard the news of his father's murder. Laertes uses hyperbolic language in passages such as the following, where he declares he will sell his soul to the devil and toss his conscience into the pit of hell in order to avenge his father's death:
How came he dead? I’ll not be juggled with.
To hell, allegiance! Vows, to the blackest devil!
Conscience and grace, to the profoundest pit!
I dare damnation. To this point I stand
That both the worlds I give to negligence.
Let come what comes, only I’ll be revenged
Most thoroughly for my father.
This kind of exaggerated language--Laertes vows to sell his soul not just to any devil but the "blackest" devil, to toss his conscience and grace (Christian salvation) not into any pit but the "profoundest" pit of hell, and to not merely be revenged but "revenged most thoroughly" for his father's death--suggests that he is hot-headed, intemperate and ready to act without thinking. In other words, placed in the same situation as Hamlet
, faced with avenging a murdered father, he behaves quite differently. He doesn't equivocate or worry about the afterlife or experience suicidal ideation. Filled with rage, he simply wants blood, no matter what the consequences. While Hamlet
, even though feeling deep grief over his father, is careful not to let the ghost
manipulate him lest the ghost be a demon tempting him to murder an innocent man, Laertes's temperament might be summed up as "act now, think later." This makes it easier for Claudius to manipulate him.