Being a Jerk is Unethical
Part of being an ethical person is treating others with basic, not superficial, courtesy.
Judging from what I've seen in the workplace, a surprising number of people would disagree with that. They think ethical behavior = honesty and integrity and not a whole lot beyond those two. Being courteous or nice is seen as a fluffy option and not an ethical requirement.
Consider, however, the effects of unpleasant behavior. It fosters acrimony and distrust, divides teams, lowers productivity and effectiveness and, by doing all of those, squanders the resources of the employer. A person who intentionally wasted company supplies or broke company equipment would be regarded as treating those items in an unethical manner. Why shouldn't someone who is abusive to co-workers be seen as unethical?
Surveys have indicated that most Americans regard respect, concern for others, and fairness as important ethical values. The jerks who are technically honest but personally obnoxious need to expand their view of ethics.