However, a second half penalty from Troy Deeney restored parity.
Brazilian starlet Richarlison went down under pressure from Hector Bellerin in the Arsenal box and the referee duly pointed to the penalty spot.
I have to watch what I say but it's having a bit of cojones, I think the word is, having a bit of nuts.
Unity deal offers hope for Palestinians
It has fought three wars with Israel since 2008 and the blockaded Gaza Strip has seen deteriorating humanitarian conditions. Palestinian President Mahmoud Abbas had previously insisted no unity government could be established before Hamas disarmed.
Now it is Arsenal whose "cojones" are the subject of scrutiny, after Troy Deeney spelled out bluntly what opponents and believe and the club's supporters fear - that Arsene Wenger's men don't have the appetite for a scrap. Whenever I play against Arsenal, I'll go up and think, "Let me whack the first one and see who wants it".
"Last time I was here I scored a few goals and you get a real buzz but a last-minute victor against Arsenal.it was definitely worth the wait for my goal since I have come back", he added.
"I nodded it down, the crowd gets up, "yeah we've got somebody who can win it", and they all just backed off".
Watford's Richarlison wins a penalty against Hector Bellerin - does he dive? "Since the first day we have worked on that idea, and I pass one idea onto my players".
He added: "I just think personally there's an element of that being an equaliser for me. That's my strength, so if you're going to let me do that against you, you're in for a tough afternoon".