I have a friend whose husband badly beat her. I'm going to court with her tomorrow. He says he will come after me if I do this.?

Report it to the judge when you testify also report it to the police before court. REPORT. Also make sure you in fact testify, fear of Domestic abusers use this tactic most times. She and you also need to get a restraining order at the same time. Court normally will give you and she restraining orders once the violence has occurred and a criminal case is going on.

Report him and his threats to the police.

Tell him you’ll shoot him if he does

You tell the lawyer every word of his threat.

Yes, as someone said, go to the court officials with this witness tampering information.

You are a good friend and unless people stand up to that bully, he will not only get away with assaulting women again, he could possibly murder someone. When he realizes the authorities are onto him, like most bullies  he will deflate and retrieve by leaving you alone. You and your friend should be carrying pepper spray at all times.

Because if you have to use it, the law will be on your side due to his domestic violence history.

Ok, in every state, this is a crime and a pretty serious one.  It's witness intimidation and/or witness tampering.  Since you're doing something so helpful for your friend, she can help you by getting in touch asap with the DA, the prosecutor, the victim's advocate or whomever she has been working with closely.  By the time he appears before the judge, this should be an additional charge against him.  But it's mandatory this happen before the court hearing.  They can't add charges in the middle of a hearing.  So make sure she does this for you immediately. 

Whether or not he'd come after you is a separate issue, because odds are high he won't.  But it needs to be on record and you probably need a restraining order, as well.

Yes, I believe that's what he said.

Add witness intimidation to the charges.

He is threatening you because he wants to invoke fear in you. He probably won't retaliate, but if you fear him, tell the police. It is against the law to threaten someone as he did you. 

You need to report any threats to the police straightaway.