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 implies what we already know about Laertes, that he is rash. He does things without thinking, and he says things without thinking as well. He acts promptly at hearing about the death of his father. But though he is rash and eager for revenge, he is not smart enough to avoid being outwitted by Claudius. Claudius turns his anger into a tool to get rid of Hamlet.