Also I think if a cop did what Zimmerman did, he would at least lose his job.
Ummm, depending on how things actually went down.......no.
But that's irrelevant, a cop's presence changes the dynamics of the situation. You cannot simply swap out Zimmerman for a cop and analyze and compare from there, because it's not a 1:1 comparison.
Put a uniformed officer following Trayvon that night, instead of Zimmerman, and you likely don't have an attack at all, because Trayvon could see that a cop is a cop, versus having no idea who the fuck is following him when it's Zimmerman (and feeling plausibly threatened, as a result).
But fine, let's run this through with a set of assumptions, in spite of the unknowns:
A uniformed officer is engaged in the lawful execution of his duties (ie. investigating suspicious circumstances at night) and follows Trayvon. Maybe catches up to Trayvon and says "police! What are you doing out here?" Trayvon decides to attack the officer, gets the better of the cop, wailing away on him to the point that the officer is fearing for his life, and he shoots him.
No, the cop would not lose his job. That would be considered a justified use of force.
But as I said, it's not really an accurate comparison to make like that. Officer presence alone changes both the subject's perceptions and reaction, and changes the "objective" context of a situation (ie. with officers having the authority, and responsibility to engage in actions that people like Zimmerman don't, and shouldn't be)