Then... why would he tell the cops about the booby-traps ?
Did he not want to kill any LEO's ? Is that why he just walked out to the parking lot, and surrendered ?
The explosives were planned to go off before his attack. It was meant as a distraction. He thought that most of the cops would be called to the scene of the huge explosion he had planned and this would leave the police unable to get to the theater when he started shooting.
Fortunately, the next door neighbor did not open the door all the way, so the explosives never went off, and the police were therefore at the theater when he started shooting. In addition his gun jammed so he didn’t get to do all the damage he had hoped to do.
He surrendered because there was nothing else he could do. He obviously wasn’t suicidal, but just wanted to kill others. And he was probably taunting the police when he told them about rigging the apartment. He would have known perfectly well that the police don’t just rush into an apartment from which a mass murderer has come without first checking for booby traps, so he probably didn’t have much hope that he was going to be able to kill any of them.
In any case, the whole point of his attack was precisely to kill the vulnerable and defenseless and show what a cruel, evil Joker he was.