Is an employee's work as a consultant or trainer for a competing company, utilizing their accumulated technical expertise, considered a betrayal of trust and deception, even with their commitment not to disclose private information about their original company?

There is no objection for the questioner to work for another company outside of his official working hours for his company. A private employee may work as he wishes outside his official working hours, provided he does not disclose his company's secrets or use its special work tools without its permission.