What If... #52: Bouncy Balls stipulates:
If you wanted to be sure of killing someone, you'd need a lot more balls. 3,000,000 of them—enough to fill a large room—would be be enough to guarantee that the target would either be crushed to death by the impact or buried too deep to dig themselves out.
To your last question, if someone just happened to walk underneath when you dropped the bouncy balls, and they were killed by the impact, you'd most likely be guilty of some form of manslaughter.
However, by asking this question, you've shown your intent to cause harm to the victim, demonstrating clear malice aforethought. By writing in to this blog, you've probably upgraded your charge to murder.
Assume we are in Utah and repeat their Geekweek experiment with the calculated 3-million bouncy balls, set up to fall in a way that it could do what Randall calculated and lead to the death of a passerby. Did we satisfy murder by having read his calculation? What if we had sent in the question in the first place?