Welcome to the forum @Annon111
Sorry to hear of your experience - a dog that has previously been aggressive is not permitted on THS so yes you could have left the sit for your safety.
“5.2.4. ensure that no pets to be left under the care of a Sitter have ever caused a person or animal any physical harm (no matter how minor the injury);”
“5.2.11. not have any inherently dangerous pets ….. or any animal with a history of attacks on pets or people;
Whilst on a sit or incase of any urgent situations you can call the Urgent Support Phone line which is a 24/7 phone number .
You can also ask the bot to schedule a call with member services
Here is a link to the member dispute process which explains how to report the owners of this agressive pet .