What "fear of their lives" was present
I agree the shooting was wrong.
However the basis of legality is Texas law. Under Texas law no "fear of thier lives" is necessary to use deadly force against anyone commiting "malicious mischief at night" under Texas law.
Other criteria is necessary.
Texas has strange criteria for deadly force in that regard. For example under Texas law if someone commits theft during the nighttime, is escaping with that property, and the person reasonably believes the individual will get away and that property will be lost they can shoot the individual in the back to prevent escape with that property. They are under no threat of harm, and can legaly use deadly force just to prevent escape with any stolen item. The escaping individual could be running down the block 200 yards away.
Whether it is a cheap outdoor decoration, the person's vehicle, or some medication they will die without it is theft during the night, and the property may not be recovered if the person escapes.
Under Texas law deadly force then becomes legal.
You won't find that in any other state.
Willfuly damaging another's property is considered criminal mischief. So willfully damaging another's property at night in Texas can allow the use of deadly force if the criteria is met.
In this case that criteria could possibly be met if the individuals felt they needed to stop the criminal mischief, or damage of the levy at night.
It may have been an ongoing problem, where people offroading night after night get away with destroying the only thing that is keeping thier property from flooding and being destroyed.
That does not mean the shooting is moraly acceptable, but it may be legaly acceptable under Texas law:
§ 9.42. DEADLY FORCE TO PROTECT PROPERTY. A person is justified in using deadly force against another to protect land or tangible, movable property:
(1) if he would be justified in using force against the other under Section 9.41; and
(2) when and to the degree he reasonably believes the deadly force is immediately necessary:
(A) to prevent the other's imminent commission of
arson, burglary, robbery, aggravated robbery, theft during the
nighttime, or criminal mischief during the nighttime; or
(B) to prevent the other who is fleeing
immediately after committing burglary, robbery, aggravated
robbery, or theft during the nighttime from escaping with the
property; and
(3) he reasonably believes that:
(A) the land or property cannot be protected or
recovered by any other means; or
(B) the use of force other than deadly force to
protect or recover the land or property would expose the actor or
another to a substantial risk of death or serious bodily injury.
That does not mean the outrage after such a callous shooting will not result in charges being filed. The outcome under Texas law though should be based on the law if the Grand Jury or later a jury are judging the actions on the basis of the law and not the morality of the action.