I always saw Illusive Man as being True Neutral, actually. The only thing he wants is his race, his group, on top at any cost. Within those parameters, he's quite unpredictable. When will he support you? When will he betray you? You never know. Remember: not all villains are evil. Being a villain, or an antagonist if you prefer, just means opposing the goals of the protagonist.
And while we're on that subject, Paragon doesn't mean good, and Renegade doesn't mean evil. The way I prefer to define them is as heroic and anti-heroic, respectively. No one would say that Wolverine of the X-Men is evil, but neither would anyone doubt that he's a straight-up Renegade.