Allah, Subhanna wa Ta'ala, knows well if it was an accident or not and Allah, Subhanna wa Ta'ala, is the Most Merciful and the Forgiving. Alhamdulliah!
Insha'Allah, it is not "okay" to murder unbelievers. Muslims are taught that to murder one person is the same as if one murdered every person.
Instead of listen to others who have distorted Islam, its teachings and tenets and who make up lies about the Prophet Muhammad, salla Allahu 'alayh wa salaam, , why not read the Holy Qur'an and learn about the life of the Prophet?