I Am Death Become Ye
He wasn't just useless; he was a liability. He was half-crazy, on medication for a whole list of mental illnesses, and physically crippled. He walked with a cane carved from old wood and sources referred to his left eye as "unreliable." Poor, fragile little human. They left a rabbit on his doorstep, just to make sure he got the message; he wasn't necessary in this game. He was boring. Oh he sounded interesting at first: humans who ran with wolves always did. But he was just another stubborn fool, and a broken one at that.
OR some fae make a mistake; Stiles teaches them the error of their ways.