Why was general shermans army so harsh as it marched through South Carolina?

Because it was attacking the civilian infrastructure that supported the Confederate troops in the field. The sooner the South surrendered, the sooner the campaign would stop.

Georgia had quit the United States in an arrogant spirit, with dancing in the streets, and now Grant had told Sherman to "Make Georgia howl".

Naturally it looked harsh when the armies had been told to blow-up railroads and burn down farms. But violence against civilians had been strictly banned, and when this did happen, it was usually carried out not by Sherman's men, but by the lawless mounted vandals (mostly deserters from both sides) who rode alongside the army for the pickings and the fun.

(Addition by same respondent)

Sorry, I thought you meant Georgia.

When they crossed into South Carolina, they put boot in, because S.C. had been the first state to secede. They were the ones who had started all the trouble. (They had done even more dancing in the streets.)