If you think something does not exist why care if someone else does? You should pause, laugh, shake your head and move on.
Any reminder of what they already viscerally know really upsets them.
This is the same reason that the most vehement abortion supporters are those that have had abortions.
The atheists hate any reminders of God and eternal judgement.
Unrepentent pro-aborts hate any reminder that what they did was murder.
Both for the same reason.