136 The soldiers’ plan was to kill the prisoners. The issue here was not cruelty, but that the soldiers would be legally responsible if any prisoners escaped and would suffer punishment themselves. So they were planning to do this as an act of self-preservation. See Acts 16:27 for a similar incident.
137 The participle ἐκκολυμβήσας (ekkolumbēsas) has been taken instrumentally.

In other versions

The soldiers’ plan was to kill the prisoners, lest any should swim away and escape.

The soldiers planned to kill the prisoners to prevent any of them from swimming away and escaping.

The soldiers’ plan was to kill the prisoners so that no one could swim away and escape.

The soldiers wanted to kill the prisoners to make sure they didn’t swim ashore and escape.

Now the plan of the soldiers was that they would kill the prisoners lest any escape by swimming away,

And the soldiers’ counsel was to kill the prisoners, lest any of them should swim out, and escape.

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