The email was March 2. Benghazi subpoena was March 4. Is there an issue with a plan to delete something that was not under subpoena? I think the key would be to get an email after March 4 talking about deletion or to recover the 33K emails and prove that they were classified/work related.
Yes, it is illegal to delete government records, so any plan to delete emails wholesale would be destruction of government property and a violation of records retention laws, at a minimum.
The private server was created to avoid compliance with those requirements, and here we have evidence of a willful plan to destroy those documents once the private server has been discovered.