In order to keep prices for meat low in the market, livestock are frequently abused and live short, miserable lives in factory farms, which are otherwise known as confined animal feeding operations (CAFOs). It's estimated that "99% of US farmed animals are living in factory farms," which means the majority of livestock are being subject to cruelty for the sake of becoming food for humans.
Within the dairy industry, female cows are artificially inseminated and are later separated from calves at birth. Once they have stopped producing milk, they are inseminated again. Male calves are slaughtered shortly after birth for veal while female calves will be raised to become the next generation of dairy cows. 
In order to boost growth, cows are often injected with the growth hormones rBGH, which can lead to lameness and mastitis.
In the meat production factory, chickens are forced into "crowded flocks of 20,000 or more."
Chickens have a natural life span of up to ten years, but "broiler chickens are kept alive for roughly 47 days," the amount of time needed to unnaturally fatten them upward through selective breeding and growth hormones. 
Because of how fast and unnaturally they grow, debilitating injuries often result. In egg-production factories, male chicks are deemed useless and often ground up alive. Hens are forced into a battery cage with three or more other hens to lay eggs with unnatural frequency.
In the pork industry, sows are forcibly inseminated. During pregnancy, they're kept in gestation crate, which are so small that they cannot move or turn around, causing injuries including leg problems and skin lesions.
After giving birth, her young are allowed to suckle for about 17 days. After that, they're sent away to be fattened up before they're slaughtered.
Because of the close quarters these animals are kept in, they often end up trying to hurt each other. In order to prevent these injuries from happening, mutilations such as tail docking, dehorning, debeaking, and castrating are routinely conducted, usually without pain killers.
As a result of the animal cruelty that goes on within the livestock industry, promoting a vegetarian diet would help to boycott these practices. To pay money to the meat industry would mean supporting their cruel and inhumane practices.