Trying to "be the hero" is definitely the riskier option. In a "public altercation" situation, I would be inclined to be a good witness and call LE before getting involved, unless it was abundantly clear that one side was innocent and about to be seriously harmed.
In an "active shooter" scenario (statistically pretty rare), I would take a much more aggressive approach to stopping the attack. Personally, I'm willing to assume a lot of personal risk in that kind of situation...largely based on a personal grudge against active shooters and the desire to not have to see families suffer the pain that I've witnessed from such monsters. In such a situation, I have the advantage of not having small children that depend on me.