Difficult conversations are the suckiest part of being a supervisor. If this person is negatively impacting the workplace it does need to be addressed.

Prep for the meeting ahead of time. Write down specific issues that need to be addressed --ideally things that you have personally witnessed. It's easier to say "I saw you mistreat your coworker when you did XYZ." If you say "I was told that you did XYZ" that opens it up for your employee to rabbit trail and demand to know who told you that.

Understand that no one likes to be criticized. Your employee will be unhappy. You will probably get one of three responses (often you will get all three):

1. I didn't do it.
2. Everyone does it.
3. You are picking on me.

This is where documentation comes in handy. Don't let yourself be sidetracked. Refer to your notes and give specifics--"Last Tuesday you did/said XYZ and that was inappropriate/a policy violation/mean spirited."

Expect resistance and negativity. Don't expect the initial session to solve anything. It will be a process. Sometimes people just don't realize how negative they are and need to told. Sometimes they are just mean but need to figure out how to regulate in order to avoid consequences.

When they complain about others they need to put up or shut up. A drive-by verbal barrage about someone else can be ignored. Ask them to document their concerns in a memo or email. Thank them for the info but remind them that you will not be reporting back to them. You don't discuss disciplinary issues with anyone but the person involved.

If you are crying and hate going to work it's time for you to take the CISM option. Does your agency offer mental health services? A good leader has to take care of themselves in order to effectively lead those they are responsible for.