I have wondered about this myself, particularly since Ronan hasn't actually been terribly interested in these (to him) trivial characters, minor obstructions to his grand planetary revenge scheme. Nor does it make sense, since he's addressing the terrified denizens of Xandar when he uses the name, that he's referring to Starlord & Co., since why would he assume they'd know who he's talking about? Truthfully, they aren't protecting the galaxy, they're just trying to protect Xandar.
I think he's trash-talking the Nova Corps. They were the ones who killed his father, and his father before him, and etc. They're the ones who managed to get the treaty he so despises, and galaxy-guarding is the kind of thing he'd associate them with, however ironically. They have that goody-goody aspect to them.
He's telling the Xandarians that they are going to die because Nova Corps pissed him off with their cheap heroics, standing up to the might of the Kree Empire, and like that. He's not talking about the motley band of outlaws he just gave a thrashing to at all. He's not even thinking about them. His obsession is 100% with destroying Xandar to get back at Nova.
But in a few minutes, following the dance-off, he'll have that name flung back at him before he dies, and it will be associated with said ruffians by all Xandarians, who were just confused as hell by what was happening, and naturally assumed he meant the weirdos who blasted their nemesis to atoms.
It's a bit like when a British officer wrote Yankee Doodle to make fun of what he saw as the ludicrous ill-disciplined American soldiers he'd fought with in New York, only to have soldiers of the Continental Army make it their unofficial theme song. But in this case, Ronan was not even referring to the people the name ended up being stuck to. And they don't actually guard the galaxy until the second film (and the galaxy is only in danger because of Peter's Celestial heritage).
Anyway, if somebody sees James Gunn at a convention or something, ask him.