Astoria didn't kill the Roc Lord. Rather, she tossed him into a spatial dimension for him to rot until she could come back and give him a proper punishment.
Afterwards, she made her way to the abode of the Shadow Lord, head of the Invisible Panther Clan.
Now that she'd taken care of two out of the main four, Astoria had used up both the element of surprise and her free pass.
The Lightning Lord wasn't an easy character, but he died easily since Astoria used such a destructive attack at close range without giving him a single chance to dodge.
As for the Roc Lord, he was just weak. There was a reason he had to follow behind the Black Dragon Clan and accept any scraps they threw at him.
But these last two wouldn't underestimate her as they absolutely received news of her strength, and they were nowhere near as weak as the Roc Lord.