Deontology is a framework for evaluating actions. Utilitarianism evaluates actions by collapsing the distinction between actions and states of affairs, that is, we determine the goodness of an action by comparing the states of affairs that arise from taking or not taking the action. Deontology cannot be used to evaluate states of affairs, because states of affairs are not actions and do not take actions. So, deontology doesn't render verdicts on states of affairs, and therefore doesn't render the wrong verdict on states of affairs.
If the argument is that taking actions that are deontologically sanctioned brings about bad states of affairs, then you don't need to say anything more than: "Deontology positively evaluates actions that bring about states of affairs which, using a separate moral framework, I assess as bad," which you do in the first premise.
But that grants that you're evaluating states of affairs, and if you are evaluating states of affairs, then you are using (at least) something other than deontology to do so, and in your case, you are using utilitarianism.
Deontology is a framework for evaluating actions. Utilitarianism evaluates actions by collapsing the distinction between actions and states of affairs, that is, we determine the goodness of an action by comparing the states of affairs that arise from taking or not taking the action. Deontology cannot be used to evaluate states of affairs, because states of affairs are not actions and do not take actions. So, deontology doesn't render verdicts on states of affairs, and therefore doesn't render the wrong verdict on states of affairs.
If the argument is that taking actions that are deontologically sanctioned brings about bad states of affairs, then you don't need to say anything more than: "Deontology positively evaluates actions that bring about states of affairs which, using a separate moral framework, I assess as bad," which you do in the first premise.
But that grants that you're evaluating states of affairs, and if you are evaluating states of affairs, then you are using (at least) something other than deontology to do so, and in your case, you are using utilitarianism.