@feha : Summoning is normally done as a desperation move, or to procure high power living weapons.
With the living weapon scenario, getting rid of the defect makes a twisted sense, the summoned are not people to them in that scenario, and they care nothing for their ultimate fate, they are likely planning to dispose of them in the end anyway, so why keep someone around that "real people" might get attached to.
Plus even if they realize modern scientific knowledge could be valuable (which they might not, it would be to easy for them to be arrogant over their magic, and likely do not care about the peasantry living better lives) the one individual likely does not carry any information that can not be extracted from the rest.
As to morale, and keeping them in line, there are multiple ways to handle the disposal, those used to using terror and oppression will likely use the same if they think they can get away with it. But other cases they try to mask it like they seemed to be trying here. Though alot these stories have them resort to other means of control.