Three-fold:

1) Killing animals for food is something that happens in nature.

2) The purpose of this study is to inflict psychological damage. Death isn't the worst possible thing.

3) Monkeys are a lot more intelligent. Indeed, that's why the researchers are studying monkey psychology instead of pig psychology.

I'm no vegetarian either, but I think looking back 50 years from now, we'll look back regretfully at how we treat certain higher functioning animals like primates and cetaceans today.