The assistant manager's motivation is critical to determining whether the conduct is illegal. There is no law against bad management. However, if your employer discriminates or takes an adverse employment action against you based upon race, religion, gender, sexual orientation, or other protected status or retaliates against you for opposing such conduct, you may have a discrimination claim. .
Unfortunately I don't see any actionable claim here. While the manager's conduct might meet the lay definition of harassment, it doesn't meet the legal definition (which is comments or conduct targeted at members of a protected class [and based on that membership] that are so severe or pervasive that they fundamentally alter the conditions of the working environment).
The fact that he seems to treat everyone poorly also militates against your case, as it doesn't appear he is singling you (or anyone else) out for mistreatment.
I'm sorry I don't think there's a claim, but bad news is better than bad advice.
I hope this is helpful, and I wish you luck.
"Harassment," "Bullying," and "Discrimination" are words that are very often misunderstood by nonlawyers. To have a valid claim for UNLAWFUL harassment or discrimination, the conduct must be based on a protected class: Age, race, gender, religion, pregnancy, medical condition, etc. OR because the claimant asserted her/his legal rights (example: overtime pay) OR if the claimant was a whistleblower (reported illegal conduct by the employer to the government.) If your boss is just obnoxious, difficult or a jerk and his/her conduct is NOT based on a protected class, assertion of a legal right or whistleblower activity, the claimant has no claim for UNLAWFUL harassment or discrimination. Personality conflicts and working for jerks is common. Your right is to leave or try to resolve the conflict. Best wishes.