An absolutely full Paragon just walks right into joining the Reapers by the end. Uniter.
An absolutely full Renegade may as well be a Reaper (in terms of enemy status). Divider.
(By 'absolutely', I really mean everything being lower right or lower left in the wheel.)
Its all relative.
A lot of people seem to think that going upper right and lower left in convos is full Renegade. IMO it is not. You're still welcoming of others, of opportunities, of experiences.
To me, a full Renegade does not give a crap about that.
In the end, a full Renegade only has themselves. This self may be the most powerful human/humanoid in the galaxy, possibly able to claw out of completely dark rubble by themselves.
To me, a full Renegade doesn't even have a romance. Maybe has a Renegade lay with Jack - I forget how that works.
IMO ME3 encourages players to be a little of both Paragon and Renegade, compared to previous titles, but the consequences are clear when you go *100%* either way. The Paragon goes so far as to disregard there even being a war with victory conditions. The Renegade goes so far as to do anything possible to remove the Reapers from the galaxy.
Most players, due to ME3's nature (compared to ME2 especially), are somewhere very in between.
Again though, its all relative. In each scenario, a Renegade does something that:
-Removes others from the equation
-Increases personal power compared to what was in his way
All interactions, even with innocents, work that way. And yes, a full Renegade IS a bad guy. He's not uber Sith Lord, but he can get somewhat close. Conflict makes him, and those still close to him, stronger and stronger.
Now, this probably won't be enough for the Breath Scene, but that's what MP is for I guess. Its still focused on the 'war', 'conflict', 'assets'. I'd say a 100% Renegade + MP EMS = something like this

Result? A galaxy where Shepard, Alliance, humanity, organics, etc all have much more *relative* power over the rest of the galaxy than the Paragon route. Go 100% Paragon and you've make this much more complicated galaxy with many factions and the Reapers playing a significant role. Much more power is shared and likely debated on, with Shepard-Reaper authority likely being one of the only things keeping it from falling apart into chaos.
Again, obviously, most people don't go entirely either way. People may make the occasional Renegade decision enough to view their Control Shepard as not 100% acquiescent of Reaper mentality. People may make the at least occasional Paragon decision enough to imagine that their Renegade Shepard was pulled out of rubble by an ally/friend instead of clawing his way out alone.
The way that outcomes and moralities interact is pretty interesting to me.
There is such a thing as being so close to personal opinions/familiarities/traditions/kin/etc that it damages what may be considered progress. That's what Renegade can do.
And there is such a thing as being so open to other opinions/unfamiliarities/information/memories/etc that it moves you away from what was a personal or shared goal. That's what Paragon can do.
Either way, you're rewarded, because the trilogy is largely there to allow you to play out what's in your own mind (either personally or for role play). Victory or defeat is ultimately subjective here and up for you to decide. This isn't only in the ending, but in every interaction.
The thing I wish the trilogy had more of was that when we killed off characters, that most of them would still be replaced by other new characters and ones that deferred more to Shepard's will (instead of acting more independent yet friendly with him). I wanted more showcasing of Shepard being a strong individual when Renegade. This is shown in some stuff like Wreav, but I wanted much more than that. And bits where we learn of human/Alliance/Normandy presence in the galaxy being more overtly important, instead of a shared effort of Paragon. This may have included bits like news reports and convo bits of aliens giving tributes of technology, credits, etc to the Alliance instead of keeping most of it and using it themselves in the war.
Basically, you really do have to be a total jerk to honestly play TOTAL Renegade. But there's always gonna be those at least few things that keep you from going TOTAL Paragon. Eventually, some things, some scenarios.. will have you giving some sort of outburst. Whether it is a Renegade Interrupt against a Quarian admiral or Kai Leng, or something more philosophical like believing the Quarians to still be the masters of the rebellious Geth machines, or whatever. Going total Renegade may be nearly impossible to bear, but only the rare player truthfully (except when just 'auto-blue-clicking') goes total Paragon. Some rebellion (ahem, RENEGADE) kicks in and keeps them from following that single track of 'goodness'. Because really, truly great solutions aren't as simple. It is appropriate that some of the best looking ME3 outcomes require at least SOME 'Renegade' decisions - ones that show that sometimes one has to be even a little (comparatively) selfish, or conflict-oriented.
ME1-3 itself is even a big story of rebellion against the greater order set by machines, it is important to remember. They may be bigger, more structured, more intelligent, more powerful, more manipulative - but Shepard himself is the Renegade against them (except possibly a bit at the very end) that increasingly shows them that even the Reapers aren't everything they thought they were. That any 100% view of 'good' (...or 'god') is probably illusory to some extent, and needs something to challenge, and yes, fight against it. Even if that something is a scumbag himself.