Well, vigilantism itself is illegal, so yeah. But I think what people around here are concerned about is the kind of treatment that accords make possible in regard to masked vigilantes and "gifted people". Before, Matt would get a trial and probably be landed in jail. Now, it's more likely he'd just be put on the bottom of an ocean, possibly in a straight jacket, with no due process and no ending date.

We really don't know what the accords really say and who exactly do they apply to. The avengers' international endeavors are certainly the main concern, but that doesn't mean it's the sole focus of the entire law being introduced.