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:
Exercises:
Pronoun case (such as I vs. me) 1
Pronoun case (such as I vs. me) 2
Pronoun case (such as I vs. me) 3
Pronoun case: review 1
Pronoun case: review 2
Related topic:
Subject complements