The Hammers blew a 2-0 lead to lose emphatically.
Bilic refused to excuse the defensive implosion that saw West Ham's two-goal lead crumble to a 4-2 defeat.
"It's hard to explain. It was way too easy. If we defend like this we aren't going to win too many games because we scored enough and created enough," Bilic said.
"I am very angry. I am, I am. At half-time some of the players were already pale, so I didn't want to give them more criticism.
"Watford's second was a classic for YouTube because there was no communication between two players. That happens. Of course it shouldn't, but you don't expect it to happen again.
"It's more the first goal that worries me because it came from a very predictable attack. They should have to do something better to hurt us."