Do you understand the situation of Jefferson Hope in Study In Scarlet?

Hope's desire to seek vengeance is believable. That he spent everything to get it - his money, his health, years of time and effort stalking Drebber and Stangerson - is not as believable; but plausible enough to be believed. There are such possessed people.

In the story, Drebber, Stangerson and their posse abduct Lucy Ferrier, Hope's beloved, kills her father and forces her to marry Drebber. Lucy pines and dies of grief. (A healthy outdoors girl? Drebber's loathsome. He probably beat her or she took poison.) Hope vows revenge. He's a westerner from the mining camps, and in the Old West stories Doyle read, there were no police and wronged men did what they swore they'd do.

So Hope did just that.