Using viruses to treat altered or misconfigured DNA
Consider how a Retrovirus can modify existing cell DNA to 'execute instructions' on its behalf.


Not only retroviruses do that. Actually, pretty much all viruses use the machinery of the host cell on its behalf.


  Why can we not utilize lab-generated viruses to infect sick patients with a 'healthy' virus that would rewrite bad segments of DNA with something either correct or non-malus?


We actually do that. We use viruses (incl. retrovirus but not only) to insert DNA segments into eukaryote genome. See vectors in gene therapy > viruses


  Retrovirus to reverse another retrovirus


Note that I don't really understand your title so, I ignored it a bit focusing on the content of the post.