Paragon Shepard here may consider Renegade Shepard to be a willing tool when he could be more, accepting enough of his situation but working his way around and beyond it.
Renegade Shepard here may consider Paragon Shepard as in denial to his reality as a tool and his potential to be more efficient as a unit.
Renegade just gets it that Cerberus is doing X and needs Y and both is what works for him for now so he works 'for' Cerberus. It gets his objectives done.
Paragon takes this cooperation as just a part of a larger agenda of countering the Reapers. It is acceptable enough.
In both cases, working with or for Cerberus was always considered by Shepard to be a temporary status. You can be more or less sympathetic to Cerberus, but never a 'sympathizer'. You can be more or less sympathetic to Cerberus, yet always hopeful they'll at least be useful against the Reapers. In ME3 you then head back to a version of your 'old self' with the Alliance, and then either get angry at Cerberus being more useful to the Reapers than humanity (Renegade), or you are sad about them losing all potential to be useful but instead losing to the Reapers (Paragon).
Shepard is still Shepard, just different priority to things.
You could say Renegade Shepard is too trusting, but its also easy to say he just doesn't care. If Cerberus backstabs him, Renegade Shepard is more ready for the concept of being backstabbed and he has less concept of having allies so he'll just gun for Cerberus. Which Renegade Shepard is all for in ME3. It is more self-oriented, one way or another. The supposed 'falling in line' he does with Cerberus is still just his way of efficiently getting his personal objective done (though arguably he only has this objective due to outside factors, but Shepard would never think so).