The episode is suppose to make you question what you would do if you had been in Data's place. If your kidnapper announced that he would kill others if you didn't do what he told you after already killing someone, what would you do?
There's no justification for the way he acted.
Data would have been completely justified, really right from his first conversation with Kivas Fajo, in using his superior strength to take Fajo into custody. If necessary, tying him up with his own clothing, then assuming control of the ship, then communicating his situation to the Enterprise.
The ship's crew could be threaten with Fajo's being harmed, if they did not co-operate with Data. This too would have been justified by the events of the episode. Again, right from the beginning
And at the end of the episode, there is no way of telling if Data's intent was to kill Fajo, wounding
a known murder and kidnapper in order to control him would have been reasonable. A nice jagged hole burned through Fajo's shoulder? Okay.
Controlling Fajo was the key to Data's freedom.