It's very fashionable to confuse the inability to draw bright shining lines as being unable to define a thing at all, but I don't have much respect for that attitude. Of all the outcomes, "the probe engages in indefinite behavior that we are never able to classify as 'humanly benign' or 'humanly malign'" is such a low percentage that it's something I'll worry about when it happens.

The world is full of concepts we can't draw bright shining lines through. In fact the ones we can are the exceptions. We manage to have definitions even so.

Agreed. One can think of any number of actions that would be impossible to rate on a benign/malign scale. E.g. as a trivial example: aliens destroy 80% of humanity, which leads to restoration of Earth ecosystems and prevention of the inevitable future war that would destroy 100% of humanity; in 100 years humanity is in a much better position than it would have been if left alone [0] [1]

And that doesn't even include intentions. We often do bad things for good reasons, with good intentions. Malignity includes or infers the intention to cause harm. That may not be present, or the intention may have been benign.

Morality is complicated and subjective. Even judging the outcome of an action as positive or negative is complicated and subjective.

[0] I don't really want to argue whether this is true, possible, etc. Pick your own variant of example where a seemingly-malign action is actually benign in the long term.

[1] Also raises the problem of estimating "better" in this context. Exercise left for the reader.