No offense but there are all well-defined terms...
AskHistorians
Ok for a serial killer is it 2 or 3 or 10. For a terrorist is it 50 or 100. For a genocide is it like a million or similar??
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.
And does paying taxes to get people killed make you a serial killer as well?