John Wesley on Leviticus 14:5

Killed - By some other man. The priest did not kill it himself, because it was not properly a sacrifice, as being killed without the camp, and not in that place to which all sacrifices were confined. In an earthen - vessel - That is, over running water put in an earthen - vessel - Thus the blood of the bird and the water were mixed together, partly for the conveniency of sprinkling, and partly to signify Christ, who came by water and blood, 1John 5:6. The running water, that is, spring or river water by its liveliness and motion did fitly signify the restoring of liveliness to the leper, who was in a manner dead before.