Which word has the most negative connotation? unfortunate, regrettable, inconvenient, or disastrous?

Disastrous has the most negative connotation because it usually involves something that has happened and there an incredible loss of life, lots of injuries or incredible property damage. The other options are usually something that will also pass easily where disastrous usually means it will take time to recover or fix the things that have happened.

Nurse can be a person or opposing or hostile to another person

Afirefighter is a rescuer extensively trained in firefighting, primarily to extinguish hazardous fires that threaten life, property and the environment as well as to rescue people and animals from dangerous situations.  just cut some parts to make them shorter and add some of your facts or what describes them most. or what they are supposed to do.