Regarding accusations of being a "shill" or "government agent", these are so common among conspiracy nuts (almost universal, even) that this may give us some insight into how their heads work.
Even someone so seriously deluded as to believe the Apollo missions were faked, JFK was murdered by the CIA, 9/11 was an inside job, or airliners are deliberately spraying toxic chemicals, must have limits to what they can make themselves believe. I know I'm not a NASA or US government employee or contractor and have never been one. Do they really think that they, an anonymous crank on Youtube, can actually make me believe otherwise?
(During college summers I did get a paycheck from the State of Maryland as a public TV broadcast engineer, but I don't count that as 'government work'. Nobody seems to consider American state and local governments and agencies to be as evil as the US federal government.)
So they must have some other reason for saying it. One possibility is that it simply makes themselves feel better. By accusing their opponents of being shills, they think they can avoid having to address the substance of what is said. This is itself a fallacy (facts are facts regardless of who says them) but fallacies are their stock in trade.
Another possibility, since these are usually public forums, is that they're just playing to the audience. People do behave very differently in public arguments than in private ones; for one thing they're far less likely to back down. But I'm pretty sure I've been accused of being a shill even in private messages.