That's an old law school hypothetical. If I push someone out the 10th storey window and you shoot them as they hurtle past the 5th storey, you're guilty of murder since you shortened the natural span of their life (which was another couple of seconds).
A guy often takes an unloaded gun and shoots at his wife while they are arguing. The son is mad at his mom so he takes the gun and loads it. After his father goes weeks without shooting at his mom he climbs to the top of the building and jumps off to commit suice=ide.as he is getting ready to jump off the dad and mom get in an argument, and on the way down the dad picks up the gun shoots, misses the wife and kills the son. To further complicate it there is a safety net a few floors down for people painting the building. The final ruling is the son killed himself.