Why is Preston so hated?

What he does in the story, trapping a girl into a marriage promise at 15 and holding her to it by refusing to accept repayment of his loan, is pretty bad, though from his own point of view, he deserves credit for being willing to wait 5 years, pass up advantageous marriages he might have made, and keeping it all secret. Besides, he really is in love, and that blind him to how badly he's behaving. However, nobody but he and Cynthia know all this, and so the question is, why is he disliked from the beginning.

If the fact that he transferred his attentions from Hyacinth to her daughter is known to the community (and it is), they might consider him unreliable where women are concerned-- especially if there have been flirtations in between, and there have (the possible specific marriages he alludes to with Cynthia and Molly). Also (and this is probably even more important), he doesn't know his place. He talks back to the Squire, he purchases good furniture (and a harp) for his home, and he clearly resents having to show deference to anybody. To our modern eyes, his resentment is not entirely unjustified.

I think we have here a complicated individual, who is vain (he is extremely handsome), ambitious, responsible in his work, and uneven in his ethics (as I stated above). But he is a man truly in love, who really has made sacrifices for the girl he wants (Cynthia hasn't got a penny, and with his looks and abilities, he could have married money). I find it hard not to sympathize with him a bit, even if he has behaved badly to Cynthia. He has wasted five years of his life dreaming only of something he was promised, and which never happened. He did, in the end, give up the letters. But in doing so, he was not just acting like a gentleman (everybody keeps reminding him he's not one by birth, but he's supposed to act like one), but giving up hope.

Molly herself knows that the letters represented his only proof of an engagement, and as such, the last shreds of the engagement itself. As long as he had them, in his mind he was still engaged. When he gives them up, he has given up. I think that's really sad. And of course the fact that I always find Iain Glen so attractive probably makes me more sympathetic.

http://www.youtube.com/watch?v=Ja-0NWXrRGA

Molly even said after he gave the letters back "He is better than I believed him to be". LOL and then Cynthia turns around and asks Molly to put herself out there to return his money. Then she runs off to London to be with "Mr. Henderson" while she is still engaged to Roger. Meanwhile Molly ruins her reputation trying to save Cynthia's. And remember Lady Cumnor read Hyacinth the riot act about Cynthia's behavior toward Mr. Preston. And then Dr. Gibson telling Cynthia how she used Molly and put her reputation at stake.

I never hated Mr. Preston like most here. I think Cynthia was just as bad and in some ways even worse.

"What happens to a dream deferred?"

Preston was not a gentleman, he was an estate manager, a position above the hired hands but below the land owners/gentry. Although he may have been educated, he had no family, no land of his own and occupied the position between the gentry and the common laborer. His best marriage prospects were governesses, successful professionals' or tradesmen's daughters/widows or housekeepers. Cynthia, as the daughter of a governess was a suitable match, as he was for her. This changed when Hyacinthe married up. He behaved badly, but so did the shallow flighty Cynthia. In the first episode he had words with the Squire over the illegal trespassing on his land which indicated he was ruthless.