I’ve recently deleted a couple of comments; on another entry. The details are unimportant. Both were by the same person, the second being an insult for deleting the first.
Here’s my rule for deleting comments: there is no rule.
I delete them at my whim. It’s my blog. I owe no explanations to anyone. In general, but not as a hard-and-fast rule, if the comment is of substantive interest to me–even if the implicit or explicit message is "Fuck You!"–you’ve got nothing to worry about.
To state it simpler: comment at your own risk.
That is all.