If you're "scary" you're scared.
"Why you so scary for?" means Why you so scared for?
"Stop acting so scary." means Stop acting so scared.
When Phat Rat called G-Unit "scary ass niggas" he wasn't saying that G-Unit frightens him..
He was saying that THEY are scared of BWS and fear them.
(BWS meaning Black Wall Street)
"Scary ass niggas" - Phat Rat on G-Unit