"If I don't then someone else will" is only an excuse for the already morally dubious. So what if someone else does it? Sure the bad thing still exists, but at least you didn't personally make the world a worse place explicitly for your own personal gain.

The unspoken part of that phrase is the second half of "so since it will happen anyway, it's not wrong for me to reap the rewards of doing the bad thing"

I hope you understand how inherently wrong that is.