He wanted to make sure nobody could shoot back and get him while he was shooting...he wanted to live to see the destruction he had wrought. Perhaps he considered that to be “enough”.
I truly think the line from the prior Dark Knight movie sums him up: “Some men just want to watch the world burn”.
He planned for the apartment to go up first, drawing the police and fire in droves, just like any city, meanwhile he is hitting the theater across town.