This is my take on a question asked on a leadership Q&A page.
There is a famous saying that goes:
"People don't leave companies. They leave bad bosses."
When it comes to finding ways to treat people poorly and lacking tact, we human beings are so innovative that it would take days to write an exhaustive answer.
We, therefore, have to limit ourselves to look at a few patterns. Maybe the most common ones.
I also want to mention that many "bad" managers are so merely because they lack interest, training, or people skills. I believe that almost anyone can learn to be a decent manager. Though for some, it'll require more effort.
Bad managers are cowards. They are quick to throw the blame on someone else. If a team member complains about the pressure, bad managers blame their superior. When their superiors inquire about the lack of results, bad managers quickly point fingers at some of their team members.
Bad managers are selfish. When their teams win, they find ways to tell the success story in a self-aggrandizing way.
Bad managers see people as resources only. They don't care about the individual behind the employee. They don't care about their needs and their wants. They don't even care about their motivation. Bad managers expect the people on their team to do the job they are asked to. Period.
Bad managers are scared. They often know that they are mediocre. It leads them to have little margin to accept errors from others. They are also terrible at absorbing pressure. Consequently, they amplify the pressure put on them, or they behave as pass-throughs. Not cool for their team.
All in all, bad managers don't know about the art and science of management. As a result, they have misconceptions regarding their job. They believe that it is to assign tasks, control the execution and verify outcomes.
Peter Drucker said that managing is:
"to make people capable of joint performance through common goals, common values, the right structure, and the training and development"
And bad managers have no clue how to use these management tools, which often require some solid leadership skills.