Here "heart" represents a person's desires. Desires are spoken of here as if they were an object that someone could set or place in a certain position. Alternate translation: "each determined to do evil to the other" (See: [[rc://en/ta/man/jit/figs-metaphor]])
This tells why their meetings will not be successful. Alternate translation: "The result of their actions will only come at the time that God has fixed" (See: [[rc://en/ta/man/jit/figs-activepassive]])