Put yourself in Laertes' shoes for a minute. Like Hamlet before him, Laertes, like any young man moving out of the house and going to college, was excited to get away from home and to be on his own for the first time. He has endured all the admonitions from his father, Polonius before leaving:
Neither a borrower nor a lender be,For loan oft loses both itself and friend,And borrowing dulls the edge of husbandry.This above all: to thine own self be true,And it must follow, as the night the day,Thou canst not then be false to any man.Farewell (1.3.75-81).
But, good my brother,Do not, as some ungracious pastors do,Show me the steep and thorny way to heavenWhiles, like a puffed and reckless libertine,Himself the primrose path of dalliance treadsAnd recks not his own rede (1.3.46-51).
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 standThat both the worlds I give to negligence.Let come what comes, only I’ll be revengedMost thoroughly for my father (4.5.148-154).
I will do't
And for that purpose I’ll anoint my sword.I bought an unction of a mountebank,So mortal that, but dip a knife in it,Where it draws blood no cataplasm so rare,Collected from all simples that have virtueUnder the moon, can save the thing from deathThat is but scratched withal. I’ll touch my pointWith this contagion, that if I gall him slightlyIt may be death (4.7.136-145).