This problem would make a nice examination question in an Evidence course.
Note 1. Does it matter whether or not the alleged killers were or were not in fact members of MS-13?
Note 2. Does the evidence show or suggest that some but not all of the killers are members of MS-13 or admire MS-13? If so, does this matter?
Note 3. Some of the evidence of an affection for MS-13 may reside on a computer web site (MySpace) and on a computer controlled by one of the killers. Does this fact implicate the First Amendment? If not, does it matter (to the question of admissibility) whether or not, e.g., the page on MySpace was seen, e.g., by its creator -- one of the alleged killers -- as a vehicle for self-expression -- or perhaps even as a vehicle for the expression and communication of political or ideological sentiments?
Note 4. Does the use of evidence about MS-13 amount to invidious profiling? Would statements by gang members in, e.g., Guatemala be admissible? If you think not, would your answer change if you found out that the alleged killer heard such statements (about, e.g, an intent to rob, torture, etc.) by the gang member in Guatemala?
Note 5. Would evidence of membership in MS-13 amount to prohibited character a/k/a propensity evidence? Or would such gang membership just amount to evidence of motive, intent, or some such thing? Does it matter what, precisely, "evidence of membership in MS-13" amounts to?
Enough questions for now. Perhaps you folks have some thoughts. (I have not even mentioned here the possibility that evidence of membership in MS-13 might be unduly prejudicial because, e.g., the evidence suggests that the alleged killer is an illegal immigrant or, say, because the gang membership evidence "reinforces" the jury's awareness of the ethnic identity of the alleged killer or killers.)