It's your blog and you have a right to allow or delete whatever you want, for whatever reason.
That said, I would recommend that before you hit "delete," you really consider whether it's your own point of view that judges that comment to be offensive, or whether it crosses a line that is offensive to most people on both sides of the argument.
As an example - and I'm not talking about your blog here - I've learned that someone, somewhere recently decided that the word "terrorist" is racist. That's an opinion that many reasonable people would not agree with. Yet the word has been censored. Another example is that it is now considered racist and hateful for a non-black person to bring up Reverend Wright's statement "God damn America" in his sermon. Again, that's a judgment not based on fact, but on some people's sensitivity based on their own opinions. Yet I've seen a discussion closed because someone merely mentioned that line - an actual quote, from the pulpit no less.
Another thing you should consider is: to what extent did your post, or another person's comment on your post, offend others or invite a controversial tone? To what extent should you take that into account before editing or deleting? I've been to a site dedicated to birth mothers who believe adoption is evil and they were wronged when they were encouraged to relinquish their children. The site is pretty hateful toward adoptive parents, and the moderator has an admitted policy of deleting any comment that doesn't agree with her. Well, at least she's honest about it. But if you have a general readership, you do a disservice if you delete honest reactions to your own biased comments, even if they are rather passionate reactions. If you don't want heated controversy on your blog, you are better off staying away from controversial topics.
I do believe there is a line people should not be allowed to cross on a public forum - at least one such as this, where the purpose is to be supportive of a variety of people. I personally haven't seen that line crossed here, but then, I don't see the deleted comments.