What is the ruling of Islamic law regarding a man who killed a boy with a gunshot to the heart, and claims that he did not intend to kill him, and that the shot was fired by mistake?

The unjustly killed is considered a martyr by some scholars. It is not permissible to exact retribution from the killer without referring to the authorities, even if they govern by man-made law, for several reasons. It is recommended to educate families to prioritize reason over emotions.