Just a quick question ...
Isnt that fact that Wyll made "a deal" with her justifying enough fot her being a Devil? O_o
I mean if she would be a Demon, she would kill him on sight ... no?
Not necessarily, no. Demons aren't all mindless killing machines, they can have long-term plans and schemes too.
The PHB description of the Fiend patron even lists a few examples of demons you can make deals with:
Fiends powerful enough to forge a pact include demon lords such as Demogorgon, Orcus, Fraz'Urb-luu, and Baphomet;