Hamlet, as part of his plan bring down Claudius and Gertrude, feels it necessary to make everyone believe he is insane, even poor Ophelia.
Instead of the tender lover she has come to know, Hamlet attacks her with vile insults and words. He accuses her of the very falseness his mother has demonstrated. In 3.1, he rails:
"...God hath given you one face, and you
make yourself another. You jig and amble,
and you (list); your nickname God's creturea dn make
your wantoness (your) ignorance. Go to, I'll no
more on 't. It hath made me mad. I say we will have no more marriage. Those that are married, already,
all but one, shall live. The rest shall keep as they are.
To a nunnery, go" (3.1.155-162).
His cruelness sends Ophelia over the edge. Without her brother Laertes to turn to, and Hamlet's scorn, Ophelia begins her descent. More tragedy for the young girl will follow, as Hamlet will murder her father, Polonius.