Two antonyms for beneficiary are giver and payer. Any word that has to do with giving or paying could also be classified as an antonym for beneficiary.
Since a beneficiary is anyone who receives benefits, profits or advantages, and an antonym is a word opposite of another word, the person who gives these benefits, profits or advantages is the opposite of the beneficiary. Therefore, the word describing this person is an antonym. As example, if a parent gives her child $5, the child is the beneficiary, and the parent is the giver. This shows that giver is the antonym of beneficiary.