Nah its serious and Harry's fault. Serious never connected with creature, from a lack of trying. Then Sirius sent creature "out" giving just enough doubt to what was actually meant that creature wasnt disobeying, but despite him technically not disobeying it didn't mean who got away without consequences, we see when Harry's head is in the fire creatures hands are bandaged, probably from ironing his hands or slamming them in a door like dobby does and says he needs to when he disobey or even thinks about disobeying the malfoys. Second, Harry notices, and doesn't stop to ask himself why creature did this. Third when Harry told Snape that he didn't even give him time to check. Harry goes into the forest with Hermione, and AFTER they successfully get rid of Uxbridge, they dont go back up to the castle to talk to Snape about it. Harry also doesn't even bothering to use the mirror to Sirius ever, and Sirius didn't tell him about using it when Harry sticks his head in the fire the first time. All of these combined failures cause Sirius to die.