What makes processed meat like salami, ham etc, dangerous to eat?

NItrites and sodium are the preservative that are not considered healthy - but the alternative before refrigeration was having no meat or eating the whole hog shortly after butchering. If you don't eat those meats frequently not really a problem - everything in moderation .

Nothing. They just contain a lot of ingredients that may be "questionable" in the eyes of the health-conscious.

Excessive fat and salt.

But why not just add salt to preserve it, as salt is a good preserving agent.

the preservatives that are added for longer shelf life.