In a translation of Lhomond's Viri Romae by Edward Roth (1898) that I came across some time ago, it says:
"Swear," he cried, "swear that you will neither desert Rome yourselves nor allow any Roman citizen to do so! Whoever does not take this oath must be informed that it is against himself that this sword is drawn." Just as much frightened as if they saw the victorious Hannibal himself, they all took the oath and surrendered themselves to Scipio to be kept in custody.
By the way, I notice that the conjoined form 'semetipsos' (they themselves) is quite common when googled. As in:
... jurant omnes, custodiendosque semetipsos
Or as in Tertullian's grisly thoughts concerning De Resurrectione Carnis:
Hi sunt, ait, qui vestimenta sua non coinquinaverunt cum mulieribus, virgines scilicet significans et qui semetipsos
castraverunt propter regna caelorum.
Obviously Hannibal wasn't the last Carthaginian to scare the pants off people.