Its hard to tell if it was really the rat king. The guy standing in the door way had a little head wrap, looked like some body armor, dingy appearance, and appeared to have vapor of some sort around him.
With Mirage its hard to tell women and human characters apart since they are drawn pretty much all the same.
The odd thing was, the cat a few pages earlier who looked up and was shadowed by something, then went running for house. Was the cat just checking out the refilled bird feeder, or was there some one there? If there wasn't anyone there, why would the cat head back to the house, if there were birds eating from the feeder?
Then when splinter was at the sink, the "rat king" if it was him, was in the doorway, shadowed, and then on the next page, Splinter goes Hmmm...either after he drank his tea or had the feeling he was being watched, and sort of did the "head straight ahead, but looked to the side with his eyes" and then in the next panel, he started to have his heart attack.
And if issues 11 cover (Splinter on a funeral pyre) is any indication, he's gone.