In Knights defense they aren't just a high frequency hedge fund that trades their own book to make a profit. They have legitimate customers that they put order

In Knights defense they aren't just a high frequency hedge fund that trades their own book to make a profit. They have legitimate customers that they put orders out on behalf of and allow clients to trade through them. Part of the reason why it took so long to shut the system down is that if they had just shut the system down, then they'd be in breach of contract for those clients. So shutting down was more of a business decision that a technical one.