The Argentina-born Chile international criticised Bilic for playing him out of position during his time with the club, as reported by Sky Sports.
The 28-year-old ended his two-year spell at Upton Park for Florence and posted a message thanking the club and the fans for their support.
But no sooner had he arrived in Italy than he was criticising everyone else for his lack of game time during his stay in East London.
After being unveiled at Fiorentina, Zarate told the Italian press: "I'm happy to be back in Italy with a great team, and to show how I can play," said Zarate as reported by Sky Sports.
"It's a good chance for me to get back on track. In England, it was good because English football is enjoyable, but I wasn't too comfortable with my role at the club and that's why I decided to come here.
"I prefer to play in a free, attacking role, one that allows me to roam."
Same old story
It was the same story when Zarate was bombed out on loan to Queens Park Rangers under Sam Allardyce last season.
Zarate said Allardyce showed him a lack of respect by not playing him ahead of the likes of Diafra Sakho and Andy Carroll.
Allardyce responded by saying Zarate 'simply wasn't good enough' as reported by the Mirror at the time.
He went on to make just four substitute appearances for QPR.
The problem with players like Zarate is that it is never their fault.
Look in the mirror for someone to blame
But instead of criticising Bilic - who has done a fantastic job at West Ham this season proving they can play good football in the West Ham way and still compete for Europe - Zarate needs only to look in the mirror for a scapegoat.
When West Ham were beset by a glut of injuries in November and December they were shorn of the likes of Sakho, Manuel Lanzini, Dimitri Payet, Andy Carroll and Victor Moses.
Pushed up front, that was Zarate's time to shine, his big chance to prove he was a forward they could rely on for goals and assists when it mattered.
But instead his profligacy cost the Hammers dear, particularly in three back-to-back 0-0 draws, in which he wasted a host of easy chances.
The skillful striker was always something of a Marmite figure among West Ham fans. Many loved watching him in action and he did bring a certain flair to the East Londoners' attack.
But others feel he simply did not pass enough, contribute enough off the ball and coupled with a tendency to fluff his lines it sealed his fate.
He did score that goal at Arsenal and a majestic free-kick against West Bromich Albion in a 1-1 draw but was too ineffective overall.
With Bilic looking to improve his striker pool, Zarate was the obvious choice to be sold.
The ex-Lazio and Inter Milan man played just 22 games for the Hammers in two years and scored five goals. With that kind of record is it any wonder Bilic was reluctant to give him the freedom he craved?