Employer gives employee 30% pay cut, they leave for a job that pays double: 'I left with zero notice'

You'd have to be pretty uncaring, have a lot of audacity, and think that you have absolute control over someone to think that they'd be willing to do the same job with a 30% cut to pay. Heck, even if the worker was willing to take the pay cut, you'd have to know that they would probably be looking for work elsewhere and reducing their output in the meantime and without a doubt you'd have to imagine that taking advantage of someone like this would certainly sour the relationship. Making your earlier thoughts of control over them wildly presumptive. 

While this would be common sense to most of us it apparently wasn't to this employer whose HR department probably looked something similar to a "surprised pikachu" meme when their worker quit with no notice after they cut their pay 30%. Their "business critical" role was left unfilled prompting HR to have to bump the pay of the role again to attract another candidate. With how much the resources of hiring cost a company it looks like they would have saved money if they had just let the worker keep their salary.