Imprecatory prayer is asking God for relief from hostile treatment at the hands of enemies. Imprecatory prayer takes the form of calling for the death or punishment of one's enemies (to imprecate is to utter a curse upon or invoke evil against someone or something).
Examples of imprecatory prayer can be found in Psalms 55:15 "Let death steal over them; let them go down to Sheol alive; for evil is in their dwelling place and in their heart", and Psalms 109:8 "May his days be few; may another take his office!"
Some people feel that imprecatory prayer isn't always Christian:
- A blogger writes, "some of them seem to go against Christian teaching (e.g. the psalmist asking God to smite his enemies in Psalm 69" 
- Expository Files - Jay Horsley