I agree with Nick. 15 litres is too small for any fish, in my opinion.
Danios are very fast swimming shoaling fish. They need to be in a group of at least 6 in a tank no less that 90 cm/3 feet in length. They will be very stressed because there aren't enough of them and the tank is too small; stressed fish behave differently from unstressed fish and could well have nipped the guppies. The guppies might have nipped each other as they too will be stressed in such a small tank.
But I also need to ask what the ammonia, nitrite and nitrate levels are in this tank. With so many fish in such a small volume, any one of those could build up quickly. Ammonia and nitrite are quick killers, nitrate is a slow killer if the level is high. And with guppies being the delicate fish they are, any water quality problems would affect them.
If you don't have your own test kit, I suggest you take a sample of tank water to a fish shop and ask them to test it for ammonia, nitrite and nitrate. get them to write down the actual number; don't accept something like 'a bit high' or 'fine', you need the numbers.
In the meantime, I would do a big water change today. That's always a good thing to do if fish have died or even just look a bit sickly.