1) Abase 2) Asperse 3) Badmouth 4) Belittle 5) Besmirch 6) Blacken 7) Calumniate 8) Defame 9) Denounce 10) Derogate 11) Insult 12) Smear 13) Traduce 1) Bad-mouth 2) Belittle 3) Blacken the name of 4) Cause to seem less serious 5) Criticise unfairly 6) Demean 7) Depreciate 8) Derogate 9) Devalue 10) Disparage 11) Lower 12) Lower the tone 13) Minimize 14) Pick at 15) Play down 16) Put down 17) Run down 18) Sully 19) To blacken 20) To make black 21) To speak damagingly of 
• (v. t.) Fig.: To blacken or sully; to defame. • (v. t.) To blacken thoroughly; to make very black.
Den'i·grate transitive verb [ Latin denigrare ; de- + nigrare to blacken, niger black.] 1. To blacken thoroughly; to make very black. Boyle. 2. Fig.: To blacken or sully; to defame. [ R.] « To denigrate the memory of Voltaire.»...
blacken belittle sully defame 
to speak damagingly of; criticize in a derogatory manner; sully; defame: to denigrate someone's character. · to treat or represent as lacking in value or importance; belittle; disparage: to denigrate someone's contributions to a project. · to make black; blacken: rain clouds denigrating the sky.
attack the good name and reputation of someone attack the good name and reputation of someone[Literary terms] charge falsely or with malicious intent charge falsely or with malicious intent