God will hold you accountable for cursing those who persecute you, when you should have been praying for them and blessing them instead. (See Romans 12:14)
Jesus said; if you are even angry with someone, you are subject to judgment! If you call someone an idiot, you are in danger of being brought before the court. And if you curse someone, you are in danger of the fires of hell. Matthew 5:22 NLT 
And I tell you this, you must give an account on judgment day for every idle word you speak. The words you say will either acquit you or condemn you. Matthew 12:36-37 NLT