It is definitely ethically unsound and it is definitely a common example even related to Nazis. Similar to "just following orders". Which I'll remind everyone, will not save you in a court of law[0]...

You are abdicating your own moral responsibility on the assumption of a deterministic reality.

The literal textbook version of this ethical issue, one you'll find in literally any intro to ethics class is

  If I don't do this job then somebody else will. The only difference is that I will not get paid and if I get paid I will do good with that money where as if somebody else gets paid they might not.

Sometimes a variant will be introduced with a direct acknowledgement of like donating 10% of your earnings to charity to "offset" your misgivings (ᶜᵒᵘᵍʰ ᴱᶠᶠᵉᶜᵗᶦᵛᵉ ᴬˡᵗʳᵘᶦˢᵐ ᶜᵒᵘᵍʰ).

But either way, it is you abdicating your personal responsibility and making the assumption that the job will be done regardless. But think about the logic here. If people do not think like you then the employer must then start offering higher wages in order to entice others. As there is some function describing people's individual moral lines and their desire for money. Even if the employer must pay more you are then helping deter that behavior because you are making it harder to implement. Alternatively the other person that does the job might not be as good at the job as you, making the damage done less than had you done the job. It's not hard to see that often this will result in the job not even existing as truthfully these immoral jobs are scraping the bottom of the barrel. Even if you are making the assumption that the job will be done it would be more naive to assume the job is done to the same quality. (But kudos on you for the lack of ego and thinking you aren't better than other devs)

[0] https://en.wikipedia.org/wiki/Superior_orders