Heathcliff is made to feel like an outsider by his own adoptive family, which fuels his desire for revenge. It's never clear where he is originally from, although Mr. Earnshaw says he picked him up in the streets of Liverpool, a port town where immigrants entered England from foreign lands. Much is made of Heathcliff's appearance by everyone who sees him; the contrast between his swarthy, brooding looks and Edgar Linton's creamy soft skin is dramatic.
So Heathcliff is a double outsider: not only is he not related to anyone at Wuthering Heights, he is also marked as racially different. He is described dozens of times as a "dark-skinned gipsy" (1.15). Even the kindly Mr. Earnshaw, when he presents Heathcliff, says "it's as dark almost as if it came from the devil" (4.45). Nelly basically sums up the regional biases of the Yorkshire inhabitants when she says, "We don't in general take to foreigners here, Mr. Lockwood, unless they take to us first" (6.7). Heathcliff's foreign appearance, although it contributes to his segregation and mistreatment, may partially explain Catherine's attraction to him.
Heathcliff is as much a threat for being foreign as he is for upsetting the family hierarchy as Mr. Earnshaw's favorite.