If everyone in the UK killed one person wouldn't the entire population be wiped out in one go?
Yes, but let's go nerd on this.
Assumptions:
1. Each person kills someone else - no suicides.
2. Each killing can happen at any time during the 24 hour period.
3. Victims are random - no organised targets.
Too late for me to do the full maths but there is a chance that any time someone kills, it is before their victim gets to kill meaning that there will be a survivor.
Worst case is that there is one survivor - everyone gets their kill In before they are killed.
Best case is 50% are survivors - nobody gets their kill in before they are killed. A kills B, C kills D etc.
As the results are statistical, I suspect on average about 25% would survive.