Reverse charge impact?

1)As we know that if forward charges is not applicable then we have to pay tax under reverse charge..why do every one say this under negative way like u need to pay tax under rcm..as we pay now and will take itc..Is there any points why people say it In negative way..

2)If on any transaction I haven't collected gst..auditor will say to pay tax under rcm..I pay tax under rcm and reduce my output in next month (itc)..by this I have nothing to lose..does rcm affect me?