That's why the Marxist critique of capital is wrong. Was use of capital to extract rent from others ethical or not? Maybe yes, maybe not, who knows. It is impossible to decide.
Georgist perspective doesn't blame the rich capitalist, but the rentier, no matter if rich or poor. Rentiers provably extract others people's and businesses' labor.
Renting out capital is not an ethical problem, but renting labor is. Human actions are not a commodity, and treating persons as tools is wrong for the same reasons “voluntary slavery” is wrong—it violates inalienable rights. The problem with capitalism is that capital owners alienate persons from their own natural property rights through the injustice of employment contracts. (Land ownership is also a problem in our world but it’s sort of orthogonal to capitalism, although it depends on how you define capitalism. Georgists make a big distinction between capital and land; maybe we can talk about both capitalism and “landism” as different strands of the economic system?)
Georgist perspective doesn't blame the rich capitalist, but the rentier, no matter if rich or poor. Rentiers provably extract others people's and businesses' labor.