what to do when your daughter's the devil?

Oh, I feel terrible! A friend of mine came over earlier in the day to keep me company as I continue to dig up the vegetable plot from hell and allow her son and Marnie to play together. This was an excellent plan and for the most part it went swimmingly. My work went so much faster with her here to talk with and Marnie stayed out of my hair long enough to actually accomplish something. I dig, Jenni talks, kids play on the swing set, babies sleep = wonderfulness. That is until Marnie pushed Noah off the end of the tunnel which is twelve feet high. On purpose. Thank God he didn't break anything. What kind of punishment does such an act call for? Seriously - that is extremely dangerous behavior. She is only three, so I am not really sure what do do beyond time outs. Nowhere in the books does it tell you what to do when your tot tries to kill her only child friend. Obviously we ended the play date and now Marnie is sequestered to her bedroom until further notice, but is that enough or appropriate?