Can I rescind a complaint to the New York State Bar that I made?

An ethics lawyer or professional responsibility lawyer would probably be the best person to answer this question, but I'll take a stab at it. I'm not sure what you told the bar, but it would seem if everything you told them was true, and or you provided them the threatening e-mail, you could theoretically contact the bar and tell them you've thought about it and don't want to proceed. If that is the case, I don't believe you could face any adverse consequences. The thing about the legal profession however is that it is self governing. So, the bar may very well decide to continue its investigation regardless. How threatening was the e-mail? That may be a factor to consider in answering the question.

No you can't. I doubt much will come of it in reality. State Bar's take with a grain of salt any complaint filed by a non-client of the attorney, and especially one that is on an opposing side of a case (I question whether the state bar is "really" looking into it, to be honest). Just like with a crime, if a rule was broken, it was broken. Your report of it doesn't change that fact.