These accords seemed to have nothing to do with registration or anything along those lines like in the comics, it didn't for example require all super-powered persons to register with the government and become defacto agents of SHIELD.
This really only seemed to be about forcing them to operate in an official capacity for the US/UN both tying their hands and putting them on leash as well. As Steve said, the "accords" meant they could be sent somewhere where they don't want to go our could *not* be sent somewhere they are needed. Since Black Widow is an Avenger, though not super-powered, she had to sign the accords in order to continue going on Avengers missions. (Otherwise I'm guessing she'd either be release/retired or would go back to work strictly for SHIELD or whatever)
It seemed to want some kind of national accountability for when shit hits the fan, like the deaths of the missionaries or whatever. Sure, we could reason that the Avengers/Stark paid reparations to the families of the deceased and provided aide to the impacted areas there's still a matter of holding someone responsible for this stuff happening and how do you do that when you're dealing with individuals acting on their own and not under a government? (I'm guessing they're protected from extradition or something so they're not held responsible as individuals, extradited, tried, and then punished.) And even with relief provided there's still a matter of punishment to prevent things happening again. Tony Stark apparently has used life's Infinite Money cheat so providing aid is nothing to him. Everyone else goes home and then to their next job. Where's the incentive or motivation to make sure they don't F-up again?
So the accords was about putting them on a leash and tying their hands so they can't just go around doing whatever.
Moreover, it's likely the deaths caused by Wanda had nothing to do with it, it was the catalyst that pushed things over the edge but given the name of the bill it's obviously something the governments have been working on since Ultron. But it was probably the government's excuse to get the Avengers under their control for their own more sinister purposes. Now they've got the Avengers in their pocket so now they can go and fight wars in the middle-east to seize natural resources, or to control third-world dictators and such.
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