There is truth here. Allowing radical jihadists into the nation will result in additional attacks that needn't have been suffered. This obviously harms the intended victims including homosexuals, Christians, members of our military, and other "infidels".
But sooner or later the public might start to notice that a disproportionately high number of attackers are of Middle Easter descent and claim to be Muslim. This might translate into something other than feelings of charity and goodwill toward peaceful American citizens who happen to share those outward traits with the terrorists.
Freely admitting unlimited numbers of those sworn to our destruction just seems a bad idea.
Of course, it seems the attacker in Florida was homegrown. But Trump never seems to get too worried about minor details.