Well, considering that antisocial is one of those terms that is used loosely in modern language without a proper understanding of its meaning (see Cloud=Emo debate) I can safely say Cecil isn't truly antisocial otherwise he would have murdered his entire party or intentionally led them all to their deaths.

In terms of whether he was a loner, I would say he wasn't. Kain is a far greater loner than Cecil and even then I don't feel the term quite fits Kain either. I feel that Cecil is a conflicted character who has willfully done a lot of things he really stood against and has a hard time sometimes accepting the terrible things he has done. I feel what I genuinely like about Cecil is that even though he is apologetic about his past actions he eventually reaches a point where he consciously tries to redeem himself. In a way he's like the anime character of Kenshin from Ruronin Kenshin who is both painfully aware of their past mistakes but holds a positive outlook towards the future and a strong will to redeem themselves by living in the present and building a better future.