In Act 4 of Hamlet, how does Laertes feel about everything that has happened?What is going on in Laertes' head during act 4?

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Jamie Wheeler eNotes educator| Certified Educator

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).
and ribbings from his sister, Ophelia, who, after he has warned her to be good shoots back: 
But, good my brother,
Do not, as some ungracious pastors do,
Show me the steep and thorny way to heaven
Whiles, like a puffed and reckless libertine,
Himself the primrose path of dalliance treads
And recks not his own rede (1.3.46-51). 
(She is saying, don't tell me to be good while you go off and do bad stuff.) 
He is blissfully unaware that Polonius has sent his servant, Reynaldo to spy on him in Paris, making 100% sure he is behaving himself and not besmirching their good name (and chances at losing the king's favor.) No, for all Laertes knows, everything is copacetic. 
He could not be more wrong. He soon gets the news that his father is dead. He assumes that Claudius is the reason. He comes back to court in a rage and bent on revenge:
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 (4.5.148-154).
It takes some convincing from both Claudius and Gertrude to make Laertes understand that it is Hamlet who has killed Polonius behind the arras, sticking him through with a sword. 
Right on the heels of this upsetting news, Laertes is stunned to see Ophelia, clearly out of her mind. She rambles on and sings, making very little sense. He learns of Hamlet's treatment of his sister and is understandably incensed. Then he discovers, via the letter Hamlet has sent Horatio, that Hamlet is on his way back to Denmark.
Given this series of events, it is easy to understand how a young man, who comes home to confusion and grief is easily manipulated by the cunning Claudius in 4.7. At this point in the play, the only person that wants Hamlet dead more than Claudius is Laertes. When Claudius reveals his multi-layered plan to assure his step-son's death, it is Laertes who offers yet a final way to assure Hamlet's death. He agrees to participate in the plan, saying
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 virtue
Under the moon, can save the thing from death
That is but scratched withal. I’ll touch my point
With this contagion, that if I gall him slightly
It may be death (4.7.136-145).