I agree that the backlash to SendGrid has been extreme, and an overreaction, but I feel that they were at least partially to blame before firing Ms. Richards. She stated the SendGrid supported her actions, and without SendGrid saying anything, they are implicitly agreeing that they support her. So I feel people have a right to be angry with SendGrid for supporting Ms. Richards, and I understand people who wrote them angry letters, or made a decision to boycott them.
On the other hand, I don't support people launching a DDoS, or any other sort of a more extreme response.