"Surely the best 'revenge' against bad people is to use their product for good things?"
No, because FOSS depends on the users supporting the developers.
The more people use a FOSS project, the more donations and visibility that project tends to receive.
And the more users that software has, the easier it is for its developers to apply for grants from charities, government schemes etc.
Using it legitimises the developers, it doesn't harm them at all.