In a 12 gallon tank the betta can really only harass a danio "just so much" because danios are way faster than the betta. At most the Betta can, like you said, make a swipe on occasion. That is not enough to cause the damage you are seeing.
However if the danio was sick and slow the betta could have exacerbated this problem by attacking him more easily. In that instance then the damage could have happened over night while the betta had a better chance to get at him. Additionally his other danio friends may have had a go at his fins as well. It is very common for healthy fish to attack sick fish.
I have 5 danios in my 10 gallon and they do really well. I would say to get your group up to at least 5. You may have room for 7 but I think that is probably pushing it.
There may not have been anything to precipitate the loss of the danio, sometimes fish just die for general reasons such as stroke, heart attacks etc. It happens to everything eventually.
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