There is no guarantee that a killer will get the DP even with a "death-qualified" jury. BTW, a death qualified jury means no one is against the DP and will consider it for the defendant, but that doesn't mean in the end they will sentence a defendant to death if, after hearing all the evidence and deliberating, they don't think it's the right sentence in that case. It's not a by-the-numbers calculation and there is subjectivity which makes the outcome uncertain until the 12 people tasked with deciding work together and decide or fail to decide. If it was simply a matter of "you commit murder you get sentenced to death" we wouldn't need a jury system. This is the system this country is founded upon. There are other countries that have a system some here are wanting (Pakistan, Afghanistan).
Arizona's bifurcated sentencing law in which another jury can be empaneled to decide death if the first jury hangs on that decision seems counterproductive and wasteful. That's the law I'd want to see changed if I lived in that state. A jury should get one shot at it and if they hang then the default is LWOP, just like it is in other states that have the DP.
One other point: a jury can't sentence someone to death because of a slimy defense strategy that tries to shift blame onto the victim. What happens as a result of defense attorneys working on a perp's case after the perp is arrested, is not part of the case for a jury to weigh. A jury can use the things said during cross-examination and the defense's case to determine if they believe the defendant or any of the witnesses, but in and of itself, 'trashing the victim' is not an aggravating factor of a crime.