Are you arguing that using the electricity you paid for is immoral, or are you arguing that using slaves is not immoral as long as you use them for something good?

I don't see how your analogy applies.

Slaves are unimportant to his point. Gold being scarce means it was hard to counterfeit gold coins in a time when you couldn't mint coins in a reliable way. Likewise, the purpose of bitcoin mining is to verify transactions in a way that is hard to counterfeit, making it more difficult to get a majority share of mining. The difficulty enforces scarcity.

Sorry, I thought it was a thinly-veiled moral argument equating bitcoin mining to use of slaves. Glad I was wrong.

Correct.