It's quite simple. My opinions can change upon learning new information, as is the case with most people.
Using the public information available to us now, it's my opinion that Zimmerman was the aggressor. We know he was chasing Martin, already making him the initial aggressor. Now, if he had witnessed Martin breaking into a house or car, chasing after him could make sense, but that wasn't the case.
So, if we learn nothing more than what we already know, I will never feel bad for Zimmerman suffering the consequences of something he is responsible for. If he doesn't chase after Martin and waits for the cops to arrive as he was instructed, chances are there is no altercation, no gunshots, no dead teenager, no endless Martin/Zimmerman threads.
However, if new information comes out which changes the scenario from Zimmerman to Martin being the aggressor, then my opinon of Zimmerman would change to reflect that new information.