If u Mass Update the internal Id to NULL will it delete all the records

You cannot change/remove the Internal Id via Mass Update (in fact, I don't think you can with any method) I have a custom Mass Update script (SS1.0) for deleting records You just have to deploy it to the record type you want to mass delete (then run the Mass Update as normal through "Custom Updates")

function deleteRecords(recordType, recordID)
	{
		nlapiDeleteRecord(recordType, recordID);
	}