Godfrey Ablewhite turns out to be the real thief of the story; he stole the Moonstone from Franklin when he realized that Franklin would not remember removing it under the influence of laudanum. Cuff, in his closing statements, speaks of Godfrey's personality, which he says has two sides:
The side turned up to the public view, presented the spectacle of a gentleman, possessed of considerable reputation... The side kept hidden... exhibited... the totally different character of a man of pleasure, with a villa... and with a lady in the villa, who was not taken in his own name, either.
(Collins, The Moonstone, gutenberg.org)
In other words, Godfrey was living two lives, one in respectable society, as a man of society, and one in a hidden villa where he kept a mistress. This latter fact, and his general ability to deceive his family and friends, is partly responsible for Godfrey accruing several large debts. It also allowed him to steal the Moonstone without guilt and arrange for it to be hidden at a bank for one year, after which he will have it cut up and sold. Godfrey seems then to be a person of impeccable reputation in society, while still able to commit acts of theft in the pursuit of his own interests. This dichotomy makes Godfrey a complex character, whose motives are larger than simple theft for monetary gain.