Hello, my dd who is also 6 has done this, when she got really cross, can't remember about what. Tbh it didn't really worry me from the point of view of it being odd behaviour ( I was keen to ensure she put it down quickly and no-one got hurt!). Perhaps I've not worried about it enough! But my view is she is too young to really understand the possible consequences of her actions and as I'm quite sure she loves her family, havn't worried about it.
When she did it I talked to her about knives being dangerous and did she really want to hurt anyone, when I know she is a kind, caring person. (She is usually, really!). She realised she was being silly and put the knife away and we had a cuddle and she calmed down.
I don't want to go on about it too much as I don't want to make the whole thing into something larger than life.
But when she's helping me in the kitchen, which she loves to do, I have majored on the dangers of knives, without mentioning her outburst.
I'd be interested to know what others think, but my opinion is that while this type of behaviour obviously needs to be addressed straight away, it doesn't mean a 6 year old has any underlying issues. I would be worried if it was recurrent, or if she was a bit obsessed with knives, but she just isn't and there has been no repeat, it happened in the summer, and she has of course got cross about lots of things since then.
Hope this helps