Is someone who invoked death upon another for a trivial reason, and then the one invoked against was afflicted with cancer and died in the same year, considered a killer?

This man is not a killer; rather, it is divine decree and destiny that coincided with his supplication. If he supplicated against the deceased unjustly, then he must repent to Allah. It is impermissible to supplicate against an oppressor with more than what his injustice deserves, based on the words of Allah Almighty: "So whoever commits aggression against you, then aggress against him in like measure as he aggressed against you." And he must seek forgiveness for the person if he wronged him with his supplication.