Internet reminds politician that no, actually, slavery was the worst

When a person must be reminded that the United States enslaved and terrorized millions of black people for hundreds of years, that person has probably said something ignorant. Offensive, even. When that person is chairman of the Prince William County board of supervisors and wants to be governor of Virginia, that ignorance is a bit more ... notable. 
Virginia gubernatorial candidate Corey Stewart, a Republican, sent a tweet on Monday that most would consider, at the very least, ill-advised.