en_tn/gen/18/20.md

the accusations against Sodom and Gomorrah

This could be translated as "the outcry against Sodom and Gomorrah." People were strongly condemning the people of Sodom and Gomorrha for their wickedness. They may have been calling on God to punish them.

their sin is so very grave

"their sin is so serious" or "they have sinned so much"

I will now go down there

"I will now go down to Sodom and Gomorrah"

go down there and see

"go down there to find out" or "go down there to decide"

as wicked as the accusations that have come to me indicate

This can also be translated as "as wicked as I have heard they are."

If not

"If they are not as wicked as the outcry suggests"