Two things.
Now, I don't intend this as an ad hominem against Wallace's biographer, but let's face it, he's fairly likely to side with Wallace on something like this. In other words, I don't think you've provided sufficient legitimate evidence that Hampden penned the letter (especially given that Hampden, as far as I'm aware, consistently denied making the death-threat).
Second, if we assume that you're correct and that Hampden did pen the letter, then as Tom pointed out, the attitude expressed in the correspondance is certainly mitigated by the horrendous misconduct and trickery of Wallace.