This evidence came out where it mattered: at trial.
A criminal trial such as this one is an event's oral recreation--not its physical reconstruction. An oral recreation can be carried out only by the living; the silent dead are at a distinct disadvantage.
We know there is evidence that the dead man had some violence in his past. We know this because the rules permit an accused man to present such evidence. The same rules generally prevent the same kind of evidence to be presented concerning the the accused man.
One man is dead, and his killer will likely see very little jail time. The jury has spoken. It's time to move on. Only a fool would more into this verdict than that. Only a fool would cheer for the killer. There is nothing to cheer about in this sorry case at all.