It is clear -let's not lie- that Slytherin is the less loved House. This has a reason, of course. I believe this reason has as much -as not to say entire- to do with Salazar Slytherin. Salazar Slytherin was a great wizard of his time, and one of the four Hogwarts founders. But Slytherin was known for only accepting half-bloods and pure-bloods in his House, not wanting any muggle-born there, as he was a muggle-hater. 

The problem of many witches and wizards is that when they think of the Slytherin House, they think of muggle-hate. It is necessary to remember that this House qualities are not those, but ambition, cunningness, determination and pride, between others. As it is to say, there's no need to hate muggles and muggle-borns to be a Slytherin. The Sorting Hat doesn't wonder whether you hate them or not at the Sorting ceremony, but wonders if you'll have what is needed.

Now, many people would say, if you asked them, that Slytherins only want to do harm, be cruel and bully people. But the truth is that the objective of this House is to achieve their means any possible way. For any matter, should the other Houses be called cruel, because they are the ones who determined we Slytherins are cruel and mean.