I think this person was out to get Gianni and Ilona!
He thought Sal was Gianni st the mall and when the shooter realized It was Sal went out and found Gianni.
I think it was someone Ilona knew JMO
Jealousy?
I don't believe your scenario works with the evidence we have to go on (so far).
Recently I went over all the clues presented. They appear to change in every media report, just so ever slightly. Also, I don't believe the witnesses ever expected to be one - so I've tossed out most of their input as non reliable nor concrete facts in any way. The scenario we're looking for must bring all three victims together at some point. The problem is, the public is assuming Gianni is alive after Ilona and Sal are discovered. I'm pretty sure Gianni was killed hours earlier. Here's why:
In my minds eye, here's what I see..
I believe this event began late at night on December 23rd. Maybe 10PM? But prior to 11:15PM.
I believe Gianni has left the gym or finished his Christmas shopping and is parked close to the spot where Ilona and Sal would be found.
As he reaches his car, he opens the trunk; to put his gym bag or presents, (or something inside), maybe even to get something out of the trunk and is immediately assaulted from out of the blue by an unknown assailant. There's a brief struggle...or maybe not. And Gianni's thrown into the trunk shot...or about to be.
[The sound of the weapon is mute from the noise of the nearby freeway, or the caliber too small to attract any attention].
The keys are hanging in the trunk - the killer now has them.
Gianni's expired.
The killer may have had the opportunity to rifle through Gianni's pockets, just briefly and may now have his cellphone(?). Who really knows except for the police.
In any case, the killer may now have become spooked, (someone's approaching from nearby or the dogs from the xmas tree kiosk area are barking) and so he closes the trunk and walks off to wait.
He's extremely nervous by now. Paranoid too.
The killer came to steal a car, (or carjack someone); maybe rob some one, or both - after hearing Macy's would be open until 2:AM - (a ridiculous hour) - just perfect to commit a crime.
At 11:15PM, the killer returns to see if anyone's noticed, or heard the shots. He's extremely paranoid by now. But he wants that car! He MUST have it! After all, he now has the keys...but some ones nearby, or he's become spooked again, (or the sick b*stard decides to do some shopping?) So he leaves. To wait for his chance to escape, unnoticed, once more.
...to be continued