They hid the evidence that was obviously detrimental to the prosecution case - if you are trying to prove somebody is innocent and was not instigator of violence, the evidence suggesting he bragged about how good he is in violent confrontation and his avid interest in topics closely associated with violence may very well make such case weaker. It's one thing saying chess club president initiated the violence and other thing saying seasoned MMA fighter initiated it (yes, I am purposely stereotyping here) - people differ with their relation to violence and how ready are they to participate in it, and evidence helps forming opinion about particular person's relation to violence. That alone would not prove or disprove it, but would be relevant and not on the prosecution's side.

Moreover, Angela Corey specifically claimed the prosecution does not have to disclose such evidence until the actual trial and can submit affidavit of probable cause based on half-truths, like it's some kind of poker game. Looks like she went further and did not disclose some of the information even after the trial started, which is clearly against the law. Moreover, if the verdict were different, it would most probably be a base for an appeal and good chance for the verdict to be thrown out due to prosecutorial misconduct and possibly another trial arranged, at great cost to both taxpayers and all participants. It is not a behavior that one has to condone in prosecutors, regardless one's opinion on the merits of the case. Even guilty ones should be prosecuted cleanly, in fair trial, not by violations of the laws and prosecutorial dirty tricks.