1. Well, I decided to consider another aorist with Christ as a doer of the action behind the verb, to get to the heart of this matter.
Here's what I came up with:
Χριστὸς ἡμᾶς ἐξηγόρασεν--"Christ redeemed us..." (3:13).
The KJV has "Christ hath redeemed us..." Here the KJV translators treated the aorist as a simple past.
2. The only problem here is that we are not dealing with a conditional sentence as in 2:21.
3. But I found a similar text at 2:18: "For if I build again the things which I destroyed, I make myself a transgressor."
4. Both 2:21 and 2:18 have the same type of conditional sentences in the Greek, with the only difference being the verb tenses in the apodosis, the then clause.
5. In 2:21 the verb is aorist, but in 2:18 the verb is present.
6. But despite the differences in the verb tenses in the apodosis in the same type of conditional sentences, the KJV treated the verb tenses as the same.