There are many possibilities around the candidate's behavior. They could be going through a manic episode, or autistic, or just a jerk, or scammer.
OP handled it well, but two things I would do differently in responding.
1. "Sorry you feel that way". I never apologize for other people's feelings, only my own actions, when am am sincerely sorry. "Not-really-apologies" are, IMO, always in bad taste.
2. Not sign emails using "best". Best what? Obviously this is up for interpretation but a dangling best is (IMO) corny and exudes "I am writing unnaturally and I think this is how professional people write"
OP handled it well, but two things I would do differently in responding.
1. "Sorry you feel that way". I never apologize for other people's feelings, only my own actions, when am am sincerely sorry. "Not-really-apologies" are, IMO, always in bad taste.
2. Not sign emails using "best". Best what? Obviously this is up for interpretation but a dangling best is (IMO) corny and exudes "I am writing unnaturally and I think this is how professional people write"