Yes, if Zimmerman had shot Trayvon in the chest for no reason, knowing that he was in no danger from Trayvon, that would count as callously gambling with Trayvon's life.
But Zimmerman did have a reason. He was in fear for his life. Therefore he was not callously gambling with Trayvon's life for no reason.
Zimmerman's reasons are irrelevant to the fact that he put Trayvon Martin's life in imminent danger, and chanced ending that life, when he shot him in the chest at close range. His reasons for doing so may affect his legal culpability for the act, but they don't change the fact
that he acted in a way that caused the death of Trayvon Martin--it is a fact that he killed Trayvon Martin.
Either Zimmerman was acting in legally justifiable self-defense, or he wasn't, when he pulled that trigger--and that has nothing to do with "gambling", callously or otherwise, with someone's life, it is really a separate issue. He shot and killed an unarmed minor--a minor who had a right to be in the area, and a minor who was not engaging in any criminal activity prior to his encounter with Zimmerman. Zimmerman claims this unarmed minor attacked him and he had to shoot him in defense of his own life.
The state can attack Zimmerman's claim of self defense based on his inconsistent statements and accounts of events, and/or on the basis of additional evidence that contradicts his assertions--and they will likely do both of those things. The state is not required to present any sort of motive for Zimmerman's actions. And the burden of proving he acted in self-defense, particularly at the immunity hearing, falls on Zimmerman.
So, the state must do two things. First they must attack and discredit his claim of self-defense, and secondly
they must try to prove he commited the crime of second degree murder by connecting his behavior to the elements of second degree murder exactly as that crime is defined by Florida law
There is a two-stage approach to the prosecution's task at trial. The two stages shouldn't be confused.