You have heard of my career and former manner of life in Judaism, how I used to hunt down and persecute the church of God extensively and [with fanatical zeal] tried [my best] to destroy it.

they only kept hearing, “He who used to persecute us is now preaching the [good news of the] faith which he once was trying to destroy.”

For if what I destroy, these I build again, I unite myself with the transgressor.

(whereas, if you snap at each other and prey upon each other, take care in case you destroy one another).