There are two things I found valuable in the letter which is why I posted it:

First an example of a notable figure, known precisely because of the habitual strength of his reasoning, refusing to engage an opponent because of the repugnance of the opponent's values and insincerity of the opponent's participation.

Second despite the (clearly evident elsewhere) power of Russell's reasoning, his justification is exclusively emotional. He feels strongly about this and he stands by what he feels. He does not attempt to downplay this motivation or launder it through reason or rhetoric.