When a pronoun functions as a subject or a subject complement, it must be in the subjective case (I, we, you, he, she, it, they).
Subject complements frequently cause problems for writers, since we rarely hear the correct form in casual speech.
He is the subject complement following the helping verb was.
If kidnapper was he seems too stilted, you can rewrite the sentence:
Exercise: Pronoun case (such as I vs. me) 1
Exercise: Pronoun case (such as I vs. me) 2
Exercise: Pronoun case (such as I vs. me) 3
Exercise: Pronoun case: review 1
Exercise: Pronoun case: review 2
Related topic:
Subject complements
subject Grammatically, a word or word group that names who or what a sentence is about.
subject complement A word or word group that follows a linking verb and either renames or describes the subject.