You can say anything about Bai Jiutian, but a fool he is not.
This situation is obviously constructed as a win-win scenario. Either manxing and his cronies kill us, or we kill manxing. Whatever happens, Bai Jiutian stands to gain from it.
However, is everything as simple as that?
Honestly, I would expect him to have another trap waiting for us. For a moment there, I thought it would make sense if he followed manxing. I do not think he would leave Lingshu with him alone, should zhang win the fight. His creepy attitude aside, if he forced her into the marriage and took control over Qingcheng, I doubt it would satisfy Bai. Similarly, if we kill him, he can take horrified Lingshu from us and unleash the pursuit. I just do not think that if we kill zhang it would be the end of it.
Furthermore, if we use our skill, I feel like it would reveal our secret to our enemies. If Bai is around, he would understand. Hell, if manxing lives, even if he does not understand what happened, he would be able to recognize Huashan (or other) techniques that no school ever taught us but that we were able to use anyway.
I don't know if this is a secret worth keeping, though. Sooner or later it will be revealed anyway.
The smart play here would be C. We don't stand to gain anything at all by staying here. If we kill him, we might lose Lingshu - and I'd rather not have her returned to Qingcheng, which might happen without us, and we play BJ's game. If we don't, we risk either our life, or our secret - which, essentially, puts our life at risk. We already know we are a far better fighter whan zhang, why would we fight him again if not to kill him.
However, I can already see
Baltika9 whipping the Codex into frenzy by screaming PUSSIES (despite pussying out himself from the most trollan choice of them all), and there is a lot of hate for manxing, so I don't have high hopes for that. I guess I'll wait and see what people vote for.