What happens if an appendicitis is left untreated?

Death. It will eventually rupture, spread infection around your belly and cause a systemic infection that can lead to death.
Rupture. If left untreated, the appendix will typically rupture within 2-3 days of the onset of symptoms. The consequence of rupture is quite variable, with some people having life-threatening infections and others developing a relatively minor infection. The data is very convincing with respect to the best management of appendicitis: remove it as soon as possible.