The helicopter was illegal or shooting it down was illegal?
We are trying to confirm this. So far, according to the reports which I have got, this is my assumption.
1. The Ukrainian Army some how got hold of an abandoned UN chopper (probably a helicopter leased from the Ukrainian Army by the UN earlier).
2. They used this chopper (without any modification or painting) to bomb the civilian areas.
3. The Donetsk People's Army downed the chopper with MANPADS.