Mean people

Today is one of those days where I'm trying to be happy despite all the chaos around me. I am a generally optimistic person and I like to give people the benefit of the doubt. But, I find that I am struggling lately with being optimistic and seeing the best in people. Just when I think that things are going pretty good, someone does something that throws me for a loop.
Last night a parent on my son's football team sent an email to the entire team giving vague references to problems he feels the team is having, and insinuating that Ben is putting the players in danger of emotional harm because of his coaching style. This same parent also gave a vague threat in this email saying that he is looking into legal action because of things he has recently learned about the team.
Ben is a VOLUNTEER head coach who stepped up to do the job when no one else would. He has been trying very hard to make it a good experience for all the kids on the team. I don't understand how people can be so darn mean! That's what it is, just mean. Whatever happened to do unto others as you would have them do unto you?????? Wouldn't life be better if we all tried harder to follow this rule?
I am far from perfect and I make my share of mistakes, but I do not ever remember a time where I was mean just for the sake of being mean.
It's just kinda been a rough couple of days...I'm sure I'll get my optimism back...I just have to realize that I can't control other people's behavior.