Translation:Your chicken is more evil than me.
I suppose 'than I am' is more correct than just 'than I', but I still think that maybe it should be an alternative answer?
I think "I" would be grammatically incorrect because it's the wrong case. "I" is for the subject of the sentence, "me" for the object.
"Your chicken is more evil than I am" or "Your chicken is more evil than me"
If you use "I" here it's still a subject, the rest of the clause is just omitted.
"I" wouldn't be wrong and should be accepted, but "me" is actually more common, because "me" in contemporary English also functions as a disjunctive, i.e. the version of the pronoun that is used when the pronoun is separated from the verb somehow. Luckily the "me" version is already accepted. ^,^
Saya is a first person personal pronoun and always refers to the speaker.