Just look at David Ayer's reaction to being chosen to direct Suicide Squad: "You can do amazing things as a filmmaker if you have the proper tools ..." Warner Bros. was going to give him exactly what he need to make the best "Dirty Dozen with supervillains" that he could. But considering that Suicide Squad was an hour of music videos followed by an hour of shooting nameless dudes, that optimism now seems obscene, like when a kid tells you that he will one day be a cowboy, a president, a paleontologist, and THE Lord of the Rings. No, you young fool. No.
The casting and pre-production process seemed to go smoothly enough, with a huge uptick in sarcastic remarks when Jared Leto appeared sporting "Damaged" across his forehead, as if he'd expected audiences to have trouble establishing the Joker's exact character. And with the exception of Leto sending his fellow cast members used Trojans to establish that he was trying to portray the Joker found in that classic Batman issue #142 "The Creepy Condom Conundrum," shooting seemed to go alright too.
"I've mailed four used condoms to prominent businessmen in Gotham, but ONLY ONE CONTAINS THE ANTIDOTE. GYAH HA HA HA!"
And then re-shooting happened, which meant that shooting didn't go alright. Or at least someone at Warner Bros. decided that it didn't go alright. Batman V. Superman had just come out, and while people praised the no parts of it at all, everyone seemed to despise the overly nihilistic tone. So they wanted to make Suicide Squad a little lighter. Add in a few jokes and haha's, because nothing improves a film like creating one and then suddenly changing its genre fucking entirely. Instead of letting the project flow naturally, they plugged it into their DC algorithm, which inexplicably churned out "3D words on screen, circle of knives, and a woman licking prison bars."