Ghosts, Horcruxes, The Veil, and Death in the Wizarding World

[Najwa]

No: HPFGUIDX 152780

Can someone help me grasp a few concepts better, because the more I
think about them, the more confused I get. 

People who die unhappy become ghosts right? Or is it people that are 
afraid to die become ghosts? If so, then why are there poltergeists 
like Peeves? Did he die in an angry manner? And if people become 
ghosts because they are afraid of passing over to the other world, 
like Sir Nicholas, then horcruxes are there for evil people to make 
and keep themselves semi-alive? 

Sirius passed through the veil, and can't come back, however if it's 
sad people that become ghosts, then surely Lily, James, and DD would 
have been ghosts for the mere fact that they did not die happily at 
all. I would think that a whole slew of the wizarding world that died 
to Voldemort and left things unsettled (i.e. The Dead Members of the 
Order) would have the urge to come back or something, however I do
think it's more that people who are afraid of passing over are the 
ones that become ghosts. 

In that case, I don't think Merope would come back as a ghost because 
she died of a broken heart and not because she was afraid to go, in 
fact she seemed to give up on life altogether. I'm just confusing 
myself with the many ways that death effects the Wizarding world. 
Anyone care to theorize on what sets all these things apart?

Najwa