Re: No Slater did not intentionally set it up.
How about set up this way then?
Monkeys can be unpredictable, they can utterly dislike some people from subtle queues in body language, I have had them run past me photographing them and attack other people. They can also do a remarkable amount of damage for their size and often one in all in with a troop.
It takes a while to get close to Monkeys in a lot of cases, depending on how used to people they are. Patience, an awareness of them and yourself around them. Traveling to Indonesia and getting to be comfortable with monkeys, and all the logistics and work that entails could count plenty as set up for me, as the above post said it's basically what things like the beeb do with their camera traps.