Well, it all depends what you mean by treatment. Hamlet himself has a very particular view of his own bad treatment at the very start of the play. He is furious that his mother could have remarried so quickly after his father's death - and to make it worse, she's remarried Hamlet's uncle (who he seems never really to have liked anyway):
O, God! a beast, that wants discourse of reason,
Would have mourn'd longer--married with my uncle,
My father's brother, but no more like my father
Than I to Hercules: within a month:
Ere yet the salt of most unrighteous tears
Had left the flushing in her galled eyes,
It's important to remember that Hamlet doesn't know at the start of the play that his uncle, Claudius, has murdered his father: 'O my prophetic soul!', he says - he knew something was wrong instinctively, but had no evidence to suggest anything was. At the start of the play, though, it's his mother who Hamlet things has done him wrong.
Once he knows that Claudius has murdered his father and stolen the throne, his list of grievances (at least as he presents them) shift in focus toward Claudius:
Why, what a king is this!
Does it not, think'st thee, stand me now upon--
He that hath kill'd my king and whored my mother,
Popp'd in between the election and my hopes,
Thrown out his angle for my proper life,
And with such cozenage...
There's a pretty good list. Claudius, Hamlet says, has (and this, by the way is at the end of the play):
- Killed his father (the king - Old Hamlet)
- Whored his mother (Gertrude)
- Stolen the throne from Hamlet himself - who might (as the son of the King) have really expected to get it next!
- Tried to kill him ('thrown out his angle' is a fishing metaphor - like the fisherman throws his line out to catch the fish, so has Claudius tried to 'catch' Hamlet's life!)
How might Hamlet's treatment be bad, then? Well, at the start of the play, he could be seen as a bit of a whiny teenager - determined that eveyrone is against him. But by the end, it's pretty clear that Claudius - who initially seems suave, nice and absolutely personable - is really trying his best to kill Hamlet off.
Hope it helps!