William made the switch from West London to North London in August 2020 as he joined the Gunners at the age of 32 on a free transfer.
Chelsea only offered him a two-year contract, while Arsenal offered the three years he wanted, making the move a no brainer as he joined Arsenal boss Mikel Arteta.
Speaking on The Obi One podcast, the Brazilian revealed that Arteta wanted him to be the main man, a promise that never worked out.
"I met them couple of times," he said. "I met Arteta, I met everyone there. And they said "we want you to be the main guy".
"Arteta said to me: "We're going to play like this and I want you to be the main guy, I will build the team around you, I want to give you the key to the club".
"When I heard that, obviously I love Chelsea but I was in the last year of my contract, I was 32, and I was thinking three more years in London, stay with my family in London, they love London. And then I decided.
"To be honest I should have stayed but at that moment I was thinking maybe the best solution is to go there, and I signed for three years there but it was not good."
Willian wished he stayed at Chelsea
Willian continued and revealed that he wished he stayed at Chelsea, even if they offered him a shorter deal in the twilight of his career.
'I got frustrated,' he added. 'I am not saying it is their fault, but when you heard a lot of things and then you join the club, things that are not the same as what they told me, I got frustrated straight away. Three months later I said to my agent I want to leave, and that I made a big mistake leaving Chelsea.'
When asked what promises were broken, Willian responded: "It was more due to what I heard. I got frustrated from that because when I was there I saw some situations that I said to myself, it's not going to change.
"But it didn't work and because of that I couldn't play well or perform because I wasn't happy. When you are not happy you cannot perform. Money cannot buy happiness. It was the best contract of my life there, but I wasn't happy."
Willian made 37 appearances for Arsenal, scoring just one goal in over 2000 minutes for the side.