Using a disjunct: I will say frankly that what he did to you is immoral but what you did to him is illegal.
Frankly, what he did to you is immoral but what you did to him is illegal.
Using a disjunct: I will say frankly that what he did to you is immoral but what you did to him is illegal.
Frankly, what he did to you is immoral but what you did to him is illegal.