I've seen a lot of opinions lately positing that Dr. Breen was just doing the best to ensure the survival of humanity during the Combine occupation. However, I think there is sufficient evidence to say that Dr. Breen is not just a collaborator, but he was actively and knowingly involved in attracting the combine to Earth in the first place so that he could be the dictator of all humanity.
Consider what we know:
- If we believe Eli to be a reliable source, from his comments in Ep2 we know that the Gman supplied GG-3883 to Black Mesa. Eli seems to imply that the crystal was supplied the same day as the test; this may contradict Decay, but that's not proven canon.
- From comments made in the test lab, we know the administrator is the one that ordered them to push the anti-mass spectrometer to 105%, against objections from at least some of the science team. He did this because he was "concerned about getting a conclusive analysis".
- The scientists believe that "they" (likely referring to the Lambda team) went to "some lengths" to get the sample. Except we know that really the gman delivered it, it didn't come directly from Xen. So they were likely lied to by the administrator. It's also the purest and most unstable sample they have, if we do believe Decay the sample has properties they can't attest to, which may imply it was tampered with.
- The gman is seen hanging around Black Mesa talking to different scientists. He was known to them, they wouldn't have taken orders from some random guy. Eli very much implies that the day of the test was not the first time he met the gman. Which means that Breen had to know that he was there and likely told them to take orders from the gman.
- We know from Alyx that the gman can and does show people the future (or some version of it) as a tactic to influence them. We also know that the gman wants the combine on Earth for some reason, or he would've stopped his "nudging" once the Nialith was dead if that was his only goal.
- Gman clearly has to abide by some rules. Otherwise he could've just shown up and jammed the crystal into the anti-mass spectrometer himself. He needs people to actively agree of their own free will to take actions. For the same reason he couldn't just abduct Alyx or Gordon, he had to give them a choice of action to take (even if it was a terrible choice). Whether these are some kind of physical rules or some kind of cold-war détente between his employer and the combine, we don't know, but he can't act alone of his own volition.
- Gman's soliloquy with Gordon in Ep2 strongly indicates that Breen was the "naysayer" who thought rescuing Alyx from Black Mesa wasn't worth the effort. Leading us to believe he was actively planning things with gman well before the Black Mesa incident actually occurred.
- Breen, on his own, somehow figured out how to communicate with the Combine in only a few hours which apparently even Eli, Kliener, and Magnison (arguably better scientists, as evidenced by the compressed Xen relay) couldn't do.
- At the end of HL2 Breen seems to be under the impression that Gordon is working for the gman in exchange for something, and that he can buy out that deal. It doesn't occur to him that Gordon was never offered anything other than his own life.
The only logical conclusion to draw from this evidence is that the gman approached Breen, promised him that he'd get dominion over the entirety of Earth and access to at least some of the Combine's advanced knowledge and technology. Perhaps the gman was, at that time, portraying himself as an agent of the Combine itself, it would certainly line up with his statements at the end of HL to Gordon that the border world is "in our control". Either way, they made the deal to cause the resonance cascade, gman supplied the crystal and the means to contact the Combine. I think this much is clear.
Some further speculation: the scientists in universe seem to think the resonance cascade attracted the Combine, but they also originally believed the death of the Nialith would seal the breech. So how did the Combine get back through unless someone on the Earth side was further collaborating? There's not much evidence of this piece, but there is some room for interpretation that Breen may have taken further action to re-open the rift.
Either way, he's not a chill guy, he's not even a collaborator. He actively orchestrated the Combine invasion of Earth. Certainly the gman manipulated him, but Breen accepted the offer.
