The point is that the one immigrant who rapes and murders a U.S. citizen didn't appear to be part of a 'one is too many' argument on your part.
Instead, that person became the "scapegoat" in the leap to "Because all migrants who commit petty crimes are likely to commit murder."
Those are two completely different contexts.
If one white male socially conservative Republican molests a little boy; that is one too many. When that happens, there is admittedly a temptation to paint all Republicans as pedophiles, but that would obviously be unfair.
In the case I noted, immigrants in general, especially unlawful immigrants, become scapegoats for the misdeeds of the few.
One unlawful immigrant who commits a brutal murder is one too many, but that should not reflect on other immigrants, lawful or unlawful.