I would agree with the general concept that an antagonist need not always be the Big Bad.
Look at DAO, for example.
Loghain was an antagonist, hands down. To some, irredeemably so. To others (such as myself), a sympathetic one. But an antagonist no less - he very much tries to kill your Warden, after all.
But the actual, mustache-twirling Big Bad of DAO was, obviously, the Archdemon, a raving, corrupted Old God trying to destroy the world in a Blight.
If that helps to illustrate the difference for anyone.
And as such, a good example with Loghain is that you could deem him beyond redemption/evil and execute him... or you could give him another chance and make him a Warden.
If they are setting Solas up as an antagonist (and not a Big Bad - Lord, I really hope they're not going down that path), I certainly hope we'll have the same option/choice on his fate as we did with Loghain. In that there was actually a choice to spare him/give him a chance at redemption.