Friendly fire should always be on, unless of course its in the tower or your home(If you can have one). It should be like in Reach where if you had been betrayed you had the option of booting that player, I think the average person has the common courtesy of forgiving a player one or twice and can tell if someone is doing it on purpose.
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I agree. Betrayals seldom got out of hand in any Halo game. It did suck when you were forced to be a man down because some griefer was put on your team and you had to boot them. That didn't happen too often, but much more so in social than ranked.