First, I think that you would need to decide what sort of a person your witness is. Is this an outsider who would be coming in and would be shocked by the violence? Or is it someone who lives in the neighborhood and has seen it all before and is jaded? Does your witness support one side or the other in this conflict?
Once you have figured this out, you have your journal entry mostly written. You need to give some details of what happened -- probably more details if your witness is an outsider (because they'd be more freaked out and likely to record everything). You need to show the incident's affect on the witness's emotions, if any. If your witness cared about who won, there would be a lot of emotion, if not, maybe the witness would only comment on how clever the "winning" sniper was.
Remember that your witness does not know that the two snipers were brothers ....