"darkgray hat" is a better term, I agree.

The points you bring up, about the expansion of what the self-appointed "fixers-of-the-net" would rationalize themselves into doing, seem quite plausible, and certainly represent a downside to darkgray hat botnets. I'm still partially convinced that they would still represent a net advantage (no pun intended) to everyone **not** infected with them -- since even if they made the infected machines unusable by their owners, as long as they made the machines unusable by other botnets it would benefit the rest of the 'net by a decrease in spam, DDOS abilities, distributed password-cracking speed, etc. And I still don't see any other feasible way to prevent computers owned by non-technical users from being used for these sorts of purposes.