"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)
Someone who does things to intentionally or unintentionally, frighten you or make you feeluncomfortable.
It can be a playful nickname for your rowdy sibling:
Sibling pretends to throw their water bottle at you.
You flinch and respond with, "Whoa Scary."
-OR-
It can be what you call someone who just... really scares you:
You're standing in line at the grocery store and you catch a hater (or one of your many haters) in the next lane glaring a hole right through to your soul, making you feel really uncomfortable. "I saw Scary at the store earlier, It was just glaring at me. Just made me feel really uncomfortable, 'bout had to have a security guard escort me out."