As far as I understand, Cas9, being given a sample of target DNA, will find a virus which DNA has a matching sample and destroy it.

Can this be used to cure all viral deceases for humans?

If a virus' DNA is known, can it be loaded into Cas9 and made into an injection for example?

The deliver is a major challenge. You cannot simply inject proteins and hope that they will act at the right area. For one, it will be too dilute and on top it will trigger immune responses. What needs to happen is that the system has to be introduced into infected cells, in which the Crispr-Cas system will be expressed. That, typically is achieved via transfection, in which a cell is infected with a virus carrying Crispr-Cas as load. There are also non-viral delivery methods in development but they all share the issue of immunogenicity, but also unintended insertions and mutations. It is still a long way until the method is suitable as an in vivo viral  therapy.

Thank you!