Look at it this way: Will all those monkeys die some day, in a few years or less?
Of course. How will they die? Disease, struck by vehicles, killed by other animals, old age, generally not pleasant.
Monkeys, unlike humans imho, have no intrinsic value, so if they can have humane deaths perhaps a bit sooner than the normal, nature's cruelty path, and also thereby improve human society, is it not a win?