Bit late to the DM party but I agree with what others have said. That DM was out of line and frankly disgusting and should not have been sent.
That being said, I also hate doxxing and Sarah is also out of line for doing that. Because let's be honest, if it had been a troll account she wouldn't have bothered. She only shared that message because it was an actual account. Her intention was to doxx, no two ways about it.
While the message was callous and rude, she doesn't deserve Sarah's stans going after her. We know what extremes they can go to. Sarah said this person was a nurse so it probably wouldn't be too hard to find where she works. This person could end up harrassed or in trouble with her job just for sending a rude message to someone online. It's not even a hate message (in the actual definition), it was just uneducated and rude. She doesn't deserve the potential IRL backlash from it. If it had been actual hate (ie; racial, homophobic etc) then yes, call her out because that's a totally different ball game.