I suggest that it is. Let's agree you are a peaceful person; now imagine the following sequence of events, which I've tried to present in the most neutral terms possible.
a. You are walking along the street, at night, talking to someone on the phone.
b. You notice me following you, driving my truck at walking pace, for at least a full minute - half a block, probably.
c. You pause and stare at me. I stare back. You see I'm also on the phone.
d. You approach my vehicle a bit more closely.
e. Something makes you decide to turn and begin running away from me.
f. You hear me get out of my truck and start running after you.
Now, just answer me one question: how would you describe your situation at this point?
Should their photos be allowed into evidence? I.e. should the jury be allowed to consider: 1) their support for a hardline right-wing sheriff; and/or 2) their membership in an armed anti-immigrant organization? Or would that bias the jury?
I get that you're trying to pose the question of whether the homicide victim was also an aggressor, but if so we need to include the context that the person in question was fleeing the defendant when the alleged aggression took place.
If I chase you down the street for no good reason, and you turn around and punch me in the nose, who's at fault?
It's my opinion that if a white person had those photos on their phone, it would count negatively against a claimed peaceful character as well.
Unfortunately whether Martin was peaceful or not is absolutely not irrelevant. An important part of a trial like this is character construction / deconstruction by the prosecution and defense.