AskHistorians

You always need a minimum of 3 occurrences to establish a pattern.

Once is a one-off

Twice is a coincidence

Three times is a pattern.

But what differentiates a serial killer from a mass shooter, spree killer, terrorist, or genocider isn't the body count. It's that they have a specific MO that they rarely deviate from.

Actually, none of the things you've mentioned are classified by body count.

A terrorist attack requires the intent to spread their message through fear. It also doesn't require any deaths.

Genocide is the intent to exterminate a specific population.