Some things are just plain wrong

Aristotle admits, however, that some things have no 'middle ground', and they are just flat out bad and should always be avoided. For instance, spite, envy, adultery, theft, murder, and so on --- these are all bad. There is no "middle ground" to these; one cannot do "just the right amount" of adultery or murder or anything like that.

Similarly, there is no middle ground for the vices, for the vices are already the extremes. Being cowardly, for instance, is having a deficiency of fearlessness. It is, then, silly to think about the "middle ground" with cowardliness. You can't be "just the right amount" of cowardly, because being cowardly is already an extreme.

Likewise, the middle ground (a skill) does not have "just the right amount" or "not enough". Having just the right amount is precisely that: having just the right amount. You can't have "just the right amount of just the right amount". Either you have just the right amount, or you don't.

So: some things are just bad, and for them there is no "just the right amount". Other things (vices) are extremes all ready, so there is no "just the right amount" for them either. Finally, other things (virtues/skills) are "just the right amount" already, so there is no "just the right amount" of them either.