How likely will someone try to kill me (on purpose or randomly) in a mass shooting?

Update your will, write your obituary, preplan your funeral, write-out all of your liabilities and assets, and check for snakes in your toilet. It's the things you never see or expect that will usually get you, like a tornado or an auto accident. The bad news is that life is terminal and you will never get out alive. Be cheerful every minute that you are alive because it will all come to end one day. If you get killed in a mass shooting you will have avoided dying from a nasty cancer and you will have made the news.