During his stint in the big house, Haigh stumbled onto the legal concept of corpus delicti, which basically means that you can't be arrested for a crime if there's no proof that crime has been committed. Haigh, being both the opposite of a lawyer and a person with more deviousness than sense, interpreted this to mean that he couldn't be arrested for murder if there was no body. By that same logic, a person couldn't be arrested for robbery if they spent all the money they stole (see "the opposite of a lawyer," above).
As soon as he got out of jail, Haigh put his spectacularly flimsy theory to the test by murdering several wealthy business owners, drinking a teacup of their blood (in addition to being stupid and lazy, Haigh was also as daffy as a shithouse rat), and dissolving their remains in acid and pouring them down the bathtub drain. After the deed was done, he would use his skills as a forger to take over his victims' businesses, using their personal wealth to finance his suddenly high-class lifestyle of fast cars, expensive hotels, and incredibly strong bathtubs.
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60 percent porcelain, 40 percent entrails
In spite of Haigh's unconventional source of tremendous income, he was still spending more than he was making, and he had to keep melting rich people to make ends meet. He'd drank at least six teacups of blood and dumped as many bodies into acid before the police finally noticed a connection between all the missing upper-class and Haigh's rapidly growing wealth, and they arrested the s**t out of him.