I don't understand how Reyes could have taken extra steps to protect the colonists or if he should have.
For one thing, Reyes couldn't KNOW that either the Shedai or the Klingons would attack the colonists. Should French and British authorities not have allowed their people to establish colonies in the Americas because they feared the Spanish or native populations would kill them?
Reyes knew there were hostile forces in the area and if the colonists had not refused UFP protectorite status, he would have made sure soldiers and starships were present to protect them. But Reyes couldn't force them to stay off that planet, nor could he force Starfleet protection upon them. If the colonists would have allowed a Starfleet presence, would Reyes have sent soldiers to a likely death to protect civilians? You bet. He would probably have gone to his own certain death to protect his ex if she had allowed him to do so. Had he knew ahead of time the Shedai would in fact attack the colonists, he probably would have broken his oath before they left as opposed to after the massacre.
Saying he used them as human shields means Reyes intentionally put the colonists in harm's way to protect his secret but he wasn't sure of any immanent danger, only a possible/likely threat. He honestly had no idea what would happen. You can't play Monday morning quarterback and judge him after the fact, you have to judge him based upon the information and options he had at hand at the time the colonists disembarked.
Reyes didn't sent the civilians to their death because he was ordered to. He allowed them to expose themselves to danger because THEY refused to both accept a non-specific warning and accept military protection.