Utilitarianism

Utilitarianism is a philosophy that seeks the greatest good for the greatest number of people, generally measuring "good" to be pleasure or happiness and "bad" to be pain. It was started in the late 18th century by Jeremy Bentham and John Stuart Mill, and its most influential modern proponent is Peter Singer.

Although it sounds good, in theory, it leads to some zany outcomes, like Ingrid Newkirk saying "a rat is a pig is a dog is a boy". Even excluding other species' happiness leads to no safety or security for the individual, because they are always subject to the whim of the greater good. You end up with totalitarianism, where property rights are completely ignored and individuals can be sacrificed merely because they are different and their annihiliation would make a large group slightly more happy.