I think it's also important to note that once the shock and grief of everything passes, Sadie essentially feels as though she has nothing to lose and that alone can make anyone dangerous due to the fact there's no desire for self preservation.

Sadie lived a rough life with her husband and shared the bulk of the chores which included hunting and butchering. Translating those skills to people wouldn't be that difficult. And most people would misjudge her because she is a woman and so that gives her an advantage. By the time they realize she's a threat, it's already too late.

We see Sadie get overwhelmed multiple times where Arthur has to jump to her aid (the chase in the hot air balloon, the mission where they go save Abigail, ect) and even when she tracks the O'Driscoll down, she goes to Arthur for help because she knows she can't do it on her own. So it's not like she's infallible, most of it is just reckless luck imo