Who are "these people"?
In what way are GCHQs coders and techies "evil"? Is it just because they have so widespread snooping powers? Is that still a problem if they have used those powers to prevent harm and injury from events that you won't have heard of? At what point does the latter outweigh the former?
I also suspect that if we follow the "GCHQ==Evil" logic, we would pretty quickly find that every coder working for a big enterprise is also "evil", and probably quite a few working for smaller ones too.
Given that it's OSS, it's not as if you're funding their vile actions by using it either.
And secondly, I'm sorry but i don't buy the slippery slope argument. As intelligent people we have clear boundaries about what is acceptable as the mandate of an organization and what isn't. If we took your approach to social issues we would never protest illegal wars because everyone else is involved in them or protest BP for polluting the Gulf because everyone drives cars. Its a ridiculous argument m