But Harry did. it's not Harry's fault for doing that though. He needed protecting then, he wasn't going to know what was going to happen in the future.

Harry could be blamed for rushing to the Department of Mysteries without properly confirming that Sirius was there. He should have spoken to Snape instead of breaking into Umbridge’s office. Snape would have been rude and sarcastic, but he would have been able to quickly confirm that Sirius was fine.

Harry was desperate to save Sirius though there were plenty of ways he could've prevented this, he could've gone to Grimald place in person to see if Sirius was there, Kreacher wasn't any help o he would have to see for himself.

Harry panicked and didn’t think things through, which is one of his character flaws. Hermione tried to make him see that there were other possibilities, but he didn’t listen.

His always wanting to do the action :P