Answer 1
Well, most Slytherins were on the side of Voldemort, who was the bad guy. Typically, bad guys don't win and aren't redeemed.
Answer 2
Each house has an attribute that defines that house, Gryffindor is honor, Hufflepuff is loyalty, Ravenclaw is intelligence, and Slytherin is cunning. There is no reason why the majority of Slytherins should by nature be violent, antagonistic, or supporters of violence and antagonism. There are many real people in the world that are defined by a strong sense of cunning and ambition, but use those attributes to argue and defend wholesomeness. It is not an outlandish question, for example, why there was no debate among Slytherins about whether they should defend the castle and protect Harry Potter? Are we really supposed to believe that every child who goes to Slytherin can at best do no harm and at worst "bring the house down"?
There are several different attempts at explaining this.
The Houses Represented Societal Groups Not Attributes:People who espouse this perspective claim that Slytherin was essentially defined by the attribute of cunning, but by choices. Most of the membership of Slytherin House was pure-blood and half-blood. Given that we know that most pure-bloods and half-bloods are aware of each other, it is likely not a stretch to imagine dinner parties where all former Slytherins would meet up and tell their children that they "absolutely must be in Slytherin because all of their friends will be". Since the Sorting Hat respects a person's choice over its own ability as a sorter, Slytherin would be directly chosen by a large number of its entrants. In order to balance the size of the houses, therefore, those who would not have chosen Slytherin, but have the attribute of cunning are less likely to be members. This allows Slytherin to become an "old boys club" with a certain mentality. Slytherin could not have been redeemed in general because the house was literally rigged to be unredeemable.
Greed and Selfishness: It may be that the very nature of Slytherin is to be selfish and cunning. This selfishness would prevent Slytherins from truly empathizing with others and therefore make Slytherins by their very nature unredeemable.
Smaller Redemption: Not every Slytherin needs to be redeemed to redeem Slytherin House. Severus Snape, Horace Slughorn, the Malfoy Family, Regulus Black, and Andromeda Tonks (née Black) were all Slytherins who each contributed in some way to Voldemort's demise. In the case of Andromeda Tonks, and Horace Slughorn to a lesser degree, we have people of good moral and desirable qualities. We also have Peter Pettigrew who is a Gryffindor who is arguably the least moral character in the entire serious. (Voldemort was amoral, not immoral.) Not everyone in Slytherin needs to be redeemed for Slytherin to be redeemed.
Slytherin
Slytherin is one of the school houses from the Harry Potter books.
The Harry potter Group Names are Gryffindor, Slytherin, Hufflepuff, Ravenclaw
The match was between Gryffindor and Slytherin.
What do you mean? The names of the four houses in Harry Potter are Gryffindor, Ravenclaw, Hufflepuff, and Slytherin.
Slytherin.
Slytherin
Harry particulary hates Slytherin.
No, James Potter was in Gryffindor house. Harry's mother was also in Gryffindor.
Slytherin is one of the school houses from the Harry Potter books.
Harry Potter was in Gryffindor, to win the house championship they had to beat Ravenclaw, Hufflepuff and Slytherin. Slytherin were their biggest competition.
The Slytherin house ghost is the Bloody Baron.
The Harry potter Group Names are Gryffindor, Slytherin, Hufflepuff, Ravenclaw
In Harry Potter and the Chamber of Secrets the password was pure-blood.
Slytherin
It is Slytherin's locket.
The match was between Gryffindor and Slytherin.