what would happen if any country accidentally changed any massive asteroid course and made it to collide with earth..?

They would probably blame it on the Dinosaurs !!

Any country (such as one with a history of nuking and firebombing civilians) who asteroided another country would risk global destruction due to the effects of the impact. They might get away with using a smaller asteroid, becasue the effects would be more localised, and the leader of the country could claim "natural disaster" as it would be difficult to prove that the incident was caused by them.

Then that country would be destroyed along with everyone else.

We would all die.

Feel free to show us how such an accident could occur...

All the dinosaurs would die. It allegedly already happened.

accidently? the same thing would happen no matter which country did it

That would violate the 1967 United Nations 1967 Outer Space Treaty big time, but humanity and civilization might not survive the impact and impact winter that would follow, so it probably would not end up going to the international courts at the Hague in Brussels. Humanity would be struggling to survive and possibly going extinct.