I think that Mustapha Mond is sort of a two part character. Personally, he is a very nice man who is understanding and open minded. This is the side of him that we see most clearly in the book. But he has another side that we don't see as much. As the World Controller, he is ruthless as he tries to maintain the society that he believes in.
Personally, Mond likes to think. He reads Shakespeare. He used to be a scientist whose work was subversive. He really understands the basis of his society and is not blindly loyal to it. He treats Helmholtz and the Savage (and even Bernard) nicely when he talks to them.
But, on the other hand, he has no qualms whatsoever about sending Bernard and Helmholtz off to exile. He takes no pity on Bernard. He is going to do what is best for the society that he rules.