I mean, he did (mostly). And he did. Of course it wasn't Jason's fault he was killed, but he absolutely directly disobeyed Bruce, and he absolutely did it thinking the Joker was in the warehouse.
We start with Bruce and Jason:
There are cars leaving the warehouse; Jason himself is the one to point out that they aren't Joker's trucks, and the dialogue here of both Bruce and Jason is very very clear that they both believe that Joker is still in the warehouse. Bruce tells Jason explicitly:
"You stay here and keep an eye on that warehouse. Take no action until I get back. I repeat: no action! ... ... Don't tangle with the Joker alone! Wait for me to get back, please! That madman's just too dangerous for you to handle. Do you read me?"
It is very, very explicit that Jason is deliberately disobeying the orders he has been given (Stay put. Do Nothing.). He knows he is; there's no other reason for him to have the thought-bubble apology.
Jason goes down to the warehouse. What exactly his plan is from there is hard to say, but at the least he was creeping along the sides of it in civilian get-up, and it appears to be luck (Watsonian perspective) that Sheila steps out and he can talk to her outside. There's no way of knowing whether it was his intention to stay outside or whether he would have gone in regardless, but he is certainly having this conversation directly outside the door of a warehouse that he still believes has the Joker in it, and the inner dialogue seems pretty clear that he went down there to save Sheila. (This part is pure extrapolation, but I personally think it's probably pretty fair to say that if Jason believed he had to break into that warehouse to save his mother, he would have done it. I don't think he went down there to stand around outside and look in windows.)
The last three panels in this page are the only ones where Jason believes to some extent that the Joker is not in the warehouse:
Sheila lies to him. She tells him Joker is gone and gets him to follow her inside, and whether it's trust or hope he does. For a few seconds, given his reaction in the last panel, he believes her, but that doesn't change anything that happened before.
Jason did deliberately ignore Bruce's orders to attempt to save his mother. He did not stay put. He did not watch the warehouse. He did not wait for Bruce to get back. He knew the danger and he did it anyway, against orders, because of course he did. That's his mother.
Jason did think he knew, right up until the moment that Sheila lied to him, that the Joker was inside the warehouse. The only action that he made believing otherwise was to follow her inside, but to say that he didn't know Joker was there is ignoring that he very much did know right up until he was told otherwise.
Jason's actions here are unfairly represented a lot of the time, especially by other characters and writers in the DC universe declaring it as 'his fault', but it is true that Jason disobeyed orders, went to save someone from the Joker, and was killed as a result of that. If he had done as ordered ā do nothing; wait for Bruce ā there's every likelihood he lives (from a Watsonian perspective).
Defend that Jason was a fifteen year old kid that had too much bravery and too much hope in his heart to ever sit on a hill and wait while his mother was in danger. Defend that he was lied to, and betrayed, and it's a narrative tragedy as designed that Jason trusted the wrong person. Don't defend Jason by disregarding the actual choices he made in this. He knew what he was doing. He knew the risk. Let him be as brave as he was.
