Theory: What if Cath took over Lancelot's body? Seeing that this would be the only way out of the situation to save Guinevere, he would give up his own body, allowing Cath to possess him and, consequently, gain the power of the Lady of the Lake. Maybe that's why Lancelot kills Ban: it wouldn't be Lancelot, but Cath seeking revenge. Or Cath could turn Lancelot into a Chaos Knight with the Chaos fragment in his body, as long as Lancelot just accepted it, just like the Four Perils