Collapse was disappointing to see, says boss
It looked set to be a second consecutive victory for the Hammers at the London Stadium, as Michail Antonio continued his excellent start to the campaign with a brace to give them a comfortable lead.
However goals in quick succession from both Odion Ighalo and Troy Deeney saw the Hornets level before the break.
Their epic comeback was sealed in the beginning of the second period with Étienne Capoue and José Holebas scoring the goals to complete the turnaround, and Bilic shared his anger at the performance of his side.
"We are disappointed and angry," Bilic told Sky Sports News HQ afterwards. "It started so well for us. For 40 minutes in the first half we were on top of the game, pressing, getting second balls, chances, hurting them behind their defence through the pace of Antonio."
Bilic continued, calling it "great" that they "scored two goals and created more chances" even though he admitted that the visitors "also had chances in the beginning."
"But after we scored the second goal it was all us," added the Croatian. "They did not put us under any pressure and then the last five minutes of the first half changed the game a lot."
Bilic bemoans Hammers' poor defending
Overall, it was a disappointing performance from the home side, especially defensively. The prime example was Deeney's equaliser before half-time, with the mix-up between James Collins and Adrian allowing the Hornets captain to make it 2-2, and Bilic was critical of West Ham's performance at the back.
"The second one, there were two individual mistakes, two very cheap goals from our point of view," he said. "We still had 45 minutes, but then in the second half we did not play like we did in the first half."
The Hammers manager bemoaned that his side "just continued to defend badly" in the second half, and said he believed that they "gave them [Watford] too much space in too many dangerous areas and it looked too easy for them from where I was standing."
Bilic concluded: "And of course, the confidence of some players then dropped enormously."
West Ham United's next game comes against West Bromwich Albion at The Hawthorns on Saturday, September 17 with kick-off at 3pm BST.