An expression used in a fit of anger when you feel exasperated, having no control of a situation or the people around you. Niggers are worthless so cussing them is an easy way to bow off steam. Just say "Kill all Niggers" to let it all out. You don't have to kill anyone. It's a joke. (at the expense of niggers).
"Damn, I'm running late. I'll never make it to this interview on time. Kill all niggers, will you?
"I'm bored. Let's kill all niggers."