Scary. Every time I've been to RFK I've taken the Metro. I've always been arriving or departing at the same time as a majority of the crowd and I've felt safe. But I wonder what I would do if the game went into extra innings, especially if it ended after the Metro had closed for the night. I've never left a game before it was over, but in that circumstance, I might have to. I wouldn't want to be walking around that neighborhood alone; I can't imagine there would be any cabs around and I doubt if I called a cab company I could get a cab to come down there late at night. I've thought about driving to the ballpark for that reason, but I'm not sure my car would be safe there either.
When the games go into extras, Nats managements calls WMATA management and they keep the trains rolling for 30 minutes after the game so you are fine as long as you don't dawdle. It will be the same when the new stadium opens next year.
MPD has a mobile command center and two smaller teams of cops equally spaced around the stadium for every game. For anyone that has any problem (lost, don't know how to get home, scared, seperated, etc...), wander over to a gaggle of cops, state your problem and they will make it go away. Really. If you are cute and young, you'll get *a lot* of attention. It you look like JimR, they'll dutifully solve your problem. Again, it will be the same at the new stadium.
DC is no different than any big city. Don't make stupid mistakes, plan ahead a tiny bit and you won't have problems.