The pairing of "minister" and "ministry" allow for a broad use of "minister" in varied contexts. Kiamlaluno's answer provides the appropriate definitions but the word can be used in a more general capacity to mean someone who administers or deals out some aspect of a ministry. This ministry is not necessarily a good or useful thing:
I am a minister of death.
The implication in this usage is someone who is above mere death and has been given the responsibility of representing or authoritatively speaking for Death. The idea of death as a ministry is a good example of the mirrored image of a government or religious ministry.
Less grim, I find these other examples apt:
He ministered to the lost
She was ministered to by the priest
This usage of a verb is not listed in the above linked answer:
verb [intrans.] (minister to) attend to the needs of (someone)
This again can be paired with a darker sense of "attending":
The grim reaper ministered to the dead.
Your mileage may vary, however. It works well with death and destruction as an intentional counterpoint to the organizations of religion and government. I wouldn't use it to describe a zookeeper feeding the ducks.