Bless them which persecute you: bless, and curse not.
Note 8 at Ro 12:14: Many people think of a curse only in relation to witchcraft. It should go without saying that Christians should not practice witchcraft against those who have done them harm. However, that is not the type of curse that is being spoken of here.
The Greek word used for "curse" here is "KATARAOMAI," and it means "to execrate; by analogy, to doom" (Strong's Concordance). The word "execrate" means "to protest vehemently against; denounce" (American Heritage Dictionary). Vine's Expository Dictionary says KATARAOMAI means "to pray against, to wish evil against a person or thing."
Therefore, vicious talk about others is actually a curse. Without realizing it, many Christians curse others and thereby allow the devil access to the lives of those they are denouncing.
Pr 18:21 says, "Death and life are in the power of the tongue: and they that love it shall eat the fruit thereof." Every word we speak releases either life or death. Our negative talk releases death. When we speak against others, we are actually releasing Satan against them.
Once we understand this, it should make us pray this prayer with David, "Set a watch, O LORD, before my mouth; keep the door of my lips" (Ps 141:3).