Mmmmm...everyone's different when it comes to their Shepard's reasons for fighting. I also don't believe it's quite so cut-and-dried as renegade = selfish, paragon = selfless. I've seen people have some pretty selfish reasons for picking Paragon options, and I know that I've had some pretty non-selfish reasons for picking the Renegade options that I do. (My Shepard's purple across the Trilogy.)
There was an art book that said it was actually mind (Renegade) vs. heart (Paragon), which seems true to me. That's probably the basis for the visual symbolism behind TIM and Renegade Shepard's eyes; it's meant to imply a cold, machine-like temperament.
Mind vs Heart isn't even the best way to look at it imo.
Renegade Shepard can be the most passionate at times, while Paragon is tepid.
It is situational. Renegade can be seen as the most logical or emotional at times. Paragon can be seen as t he most logical or emotional at times.
One way to look at it sometimes is Synthetic vs Organic, but that is just sometimes. Artificial vs Natural. Anti-Thesis vs Thesis.
Shepard comes from a place of Renegade at all times. He's an Alliance N7 soldier (regardless of 'class'). He's an organic. He must constantly prove himself to individuals and authorities. He is on a mission to destroy Reapers (even if the end goal isn't necessarily to destroy *all* Reapers, but to stop them and end the Cycle). He is either imposing or brutish-looking to the new people he meets, not automatically charming and disarming.
We then have options to embrace this place of Renegade and be the weapon we need to be against the Reapers, or to aspire for a state of Paragon and the affection from others that comes along with it. Or we can do a mixture of both, with a more customized RP experience with bits of Paragon, Renegade, and more (white options, optimized outcomes in ME3, metagaming).
Paragon is selfless in that he's prepared to die for the safety of others.
Renegade is selfless in that he's prepared to die to defeat a threat.
Paragon is selfish in that he believes himself to be the standard to follow, and may want to stick around to always have his friends.
Renegade is selfish in that he believes himself to be the individual to follow, and may want to stick around to always have his followers.
(And again, obviously there can be a mix. I am a Paragade player in ME3 who chose Destroy for the primary reason of saving others, not just killing the big robots. But I also metagamed apart from that, wanting my Shepard to survive as something as close to human as possible.)
It then depends on your own moral views on which is 'worse'. Personally, I find Renegade to be worse, but others can and have often disagreed for good reasons.
EDIT: To respond to you again, both Paragon and Renegade become more of a machine.
Renegade reveals himself to be more of a literal machine, while hating other machines for doing this to him (as he more insists on being human, which he technically still is).
Paragon reveals himself to be more of a machine sympathizer, able to actively choose to become more of a machine for the sake of others.
Renegade accepted the existing truth, while Paragon chose a new truth.
"Facing reality" vs "Find another way".