'Joker 2' Actor Behind Controversial Twist Ending Shrugs Off Fanboy Rage

"Joker: Folie à Deux" is officially DOA -- even wildly despised by some theatergoers -- but the actor at the center of one of the biggest fanboy gripes has one question ... Why so serious?

Be warned ... there are major spoilers for 'Joker 2' below!

Here's the deal ... The highly anticipated sequel starring Lady Gaga and Joaquin Phoenix only made a weak $20 million at the domestic box office opening weekend ... and even worse ... the Todd Phillips-led movie got slapped with a dismal "D" grade on CinemaScore.

Connor Storrie, who plays the perceived actual Joker -- he even cuts the smile into his face after brutally murdering Joaquin's Arthur Fleck -- tells TMZ ... the dogpiling hate is not at all surprising.

CS told us Phillips had a “big swing” vision for the flick ... and was going to see it through … irate fanatics be damned. And, he points out ... people started bitching almost immediately when there were rumors the film would be something of a musical.

Connor ... who had to keep his trap shut for 2 years, as he was only one of a handful of peeps who knew about the wild twist ending ... told TMZ he would “kill” to work with Todd again … and of course, gave some thought to a possible future of his iteration of The Joker.

As for the haters ... CS could not care less some fans are losing their minds and even using the word “betrayed” when it comes to the ending. As he sees it … that much ridiculous vitriol means the film did its job ... even if it means a box office bomb.

As for those fans pissed at him, Connor tells us … meh. He’s proud of the work -- and has heard great things from others outside the hardcore fandom.