Enchantress' brother and his armies were "well known?" I must have missed that because in the movie I watched they were brand new threats that basically appeared out of thin air.
And who SHOULD they have sent instead of Harley? The army? They did. But they didn't know if the army would be enough. So they went with the crazies as well. And do you know that even Green Berets sometimes freeze up in combat? Watch that documentary series "No Man Left Behind" if you want to see a real life example or two. Harley demonstrated time and again to US law enforcement/the government that she could clear a room of highly trained combatants without breaking a sweat AND she was guaranteed to never freeze up like that guy on the stairs in Saving Private Ryan. And these were literal monsters they were going up against, not human terrorists. As she so eloquently said in the film she's actually used to see such nightmares anytime she goes off her meds. In an insane world she was the sanest choice, it's how these movies work.
And they didn't need "origin" movies to establish these characters as villains first. Did we get prequels before the bad guys went to work in The Dirty Dozen or Escape from New York? Of course not.
I also love the notion that Batman won't ever apprehend a criminal who has a kid with him.
"New rules sir?"
"Yeah, the guy who murdered hundreds is taking a walk with his kid so I have to let him go."
"Why exactly?"
"Because of my parents Alfred, because of my parents."
"But you aren't going to murder him. You're just going to take him to jail right?"
"But...the child..."
"You dress as a ****ing BAT and literally scare the **** out of people for hours on end every night, probably quite a few kids too, are you really going to just let him go to kill and orphan hundreds more? Isn't that helping the criminals?"
"We've always been criminals Alfred."
Yeah, **** that.
I prefer:
SWOOP KICK PUNCH
Kid: "Aiiieee!"
Batman: "Sorry you had to see that kid, but your dad had it coming. His victims weren't so lucky."
*disappears*
As it stood in the actual film Batman gave Deadshot the option to go quietly so that the kid wouldn't be traumatized anyway and *he did* so even then it's a non-issue.