Why are virus all destructive in nature?

Question

The COVID-19 virus causes respiratory illnesses. The virus does so by changing the generic code of our cell.

If a virus can cause a change in our body, like respiratory illness, can the same techniques in a controlled manner be used in a constructive manner, for example treating cancer.

Simply if I ask, can we create a virus like a thing to work for good in our body, just like nuclear fusion used for bombs and electricity.

Answer 1

I think the claim about viruses always being destructive isn't a good starting assumption for a question. For several reasons:

• In the sense that OP is probably considering, it isn't true. Counterexample, see below (and there are also other uses we put viruses to).
• In a wider sense, life is always destructive in that a living organism uses energy, substances, and space that otherwise could be used by another living organism. While we could debate whether viruses are alive or not, they certainly share these points.
  So a bacteriophage (see below) may be destructive to the bacterium it infects. But that bacterium dying means the "neighbor strain" that is resistant against this phage is thriving. So one organism's destruction eases the life of other organisms.
  In that sense, I consider the claim of desctructivity with "so, what?"

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can we create a virus like a thing to work for good in our body

Have a look at bacteriophages and phage therapy.
Bacteriophages are viruses that infect bacteria, and there has been research about using them against bacterial infections about 100 years ago, in parallel to the development of antibiotics.

Besides political/language difficulties (bacteriophage research was strong in the Soviet Union, and in consquence lots of the research publications are in Russian) back then antibiotics turned out to be easier and cheaper in practice (this may change with increasing antibiotic resistances, though).

Antibiotics are produced by chemical synthesis or isolation from plant/fungi. They are chemical substances, so once we have studies that show that the stuff works as intended, product testing "only" needs to show that the correct modification of the correct substances is there in the correct concentration.

But the viruses are more-or-less alive, and in any case they evolve. So chemical substance testing is not sufficient, for each batch the biological activity must be checked and proved anew. This means a lot of expensive work for each batch of the virus.
Also, the bacteriophages are pretty much the opposite of a broad-spectrum antibiotic: they are not only species specific but also strain specfic (I don't know whether that is the case always, often, or sufficiently often to hinder practical application).