I personally am on the fence with him but I figure he is at least worth a discussion especially since he is really loathsome:
What is the Work?
Gravity Falls is a Disney cartoon about two teenage siblings that are dropped off in the towm of Gravity Falls for summer vacation. They soon realize that things aren't what they seem.
Who is he? What has he done?
Preston is a rich, snobbish and downright petty man who abuses and manipulates the lower class. While in season one he was an asshole, the second season shows him in a darker light.
He has been emotionally abusing Pacifica fir years using Pavoli conditioning as if she was a dog. While throwing a party for the wealthy when the Lumberjack Ghost who the Northwest have wronged shows up and threatens to kill all of their guests if they don't open the gates to let the lower class in. Preston just heads to the panic room like a coward and leaves all of them to die even trying to stop Pacifica from opening the gate since he values his pride over dozens of lives. Even intending to kill and eat the butler.
In the Weridmageddon three parter, he attempts to sell out all of humanity to Bill Cipher by becoming his "horseman of the apocalpyse" knowing that Bill will torment and kill everyone (including his wife and daughter) but doesn't care so long as he can save his own skin.
Played seriously?
While he is Played for Laughs at some points such as when he threatened to eat his butler. His loathesome and cowardice is for the most part more highlighted.
Redeeming Qualities?
None. He abuses his daughter and was willing to leave his family to a horrible fate at Bill's hands. He is someone who would gladly sell out even his own kind to save his skin.
Heinous Standard?
The heinous standard is really high for a Disbey cartoon. Even ignoring Bill. There is this spider serial killer named Darlene, and other dangerous supernatural beings.
However Preston is just a mere despicable human with no powers or amything but yet still manages to be worse than some of the creatures Dipper and Mable fight. While Gideon is also a mere human child who had some nasty attempted murders his evil is mostly limited to the protagonists, and he was capable of redeeming himself in the end. Preston's actions however, endangers scores of people.
The only other human villain witb a high bodycount/attempted body count is Nathenial Northwest who caused the death of the people who built the mansion due to the work conditions but he doesn't count since he is only ever seen in flashbacks for a few mins.