''Good intentions don't sanitize bad impact'' Why?

What do you mean by "sanitise"?

Anyway, the road to hell paved with good intentions.

Like cleaning

Can you be more specific? Cleaning is an action performed on physical matter. "Impact" is not a physical matter and "good intentions" is not physical either.

Why when someone makes a mistake so waht is the most important is his results (his mistake) and not his good intentions or his reasons? Why his intentions/reasons don't matter??

Because the results are the most visible thing. Death, pain, happiness, destruction, construction. The results are visible but the intentions are not. You can't read someone's mind to see his intentions. You just infer his intentions from his behaviour. Also, intentions do not affect anyone, results do. Bad intentions by themselves do nothing.