Because evil is evil, and the results can’t magically change that. If an action is evil, it ought not be done. This is pretty much a definition.
The idea that the ends justify the means is exactly the denial that any action is intrinsically good or evil - if a good effect could justify an “evil” action, then the evil action wasn’t evil after all, because the “ought not be done” does not apply.
But if there is an ultimate Good, then there is a standard by which all things, ends and means both, must be judged. If a means can be judged independent of its ends, then it can be bad independent of its ends.
And if it is bad independent of its ends, then it is actually bad and ought not be done. By definition. Regardless of its effects. Unjustifiable, by ends or otherwise.