This game has nobody declare their sole reasons for doing what they do "because I'm evil." That's simply unrealistic and doesn't work outside of Saturday Morning Cartoons.
Solus, despite his flippant personality, appears to be very much committed to the Ascians' cause. The big mystery that remains is "exactly what is their endgame and what do they mean by "return mankind to its rightful state"?" If one considers this, the only real difference between Solus and Elidibus is that the former is flippant and the latter has a tendency toward austerity - and nobody has accused Elidibus of being a "mustache twirling Saturday Morning Cartoon villain."
The Ascians may be evil from our perspective, but we still don't have the whole picture of what drives them, so it's presumptuous to simply dismiss them as evil. The closest thing in-game to evil is Zenos, and even he has motivations more complex than doing evil for evil's sake. (Not much, but even so.)
This ties in to one of the main mysteries behind Solus - we know he was an Ascian when founding the Empire, but was he one before that, and if so for how long?
I'd like it to be simple enough as just saying he was always one (and just possessed some Garlean noble to do what he did), but the revelation that people can transcend mortal limitations with or without the Ascians' gift makes it plausible that he transcended somehow and was simply convinced to support the Ascians' goals. Ultimately we don't know when he became an Ascian or if he always was one, so I feel it's too early to say a Garlean officer was swayed to their cause (because he could have always been on the Ascians' side).