It has been theorized and maybe even proven that everything is made up of energy: the solar system, the human body, even a woodworking shop.
When we add an employee, there is a shift in the shop’s energy, as everything has to move to make room for the new element. This can work well or it can totally upset the workplace.
If you hire a person who has an agenda beyond being there to work, their energy can be disruptive to the point where the productivity level is reduced rather than increased and they immediately become a liability instead of an asset. For that reason, I have always made it clear to new employees that there will be a trial period to see how well they fit in. If they don't, they have to go and the sooner the better.
Keeping someone around that is pulling everyone else down is foolish. More foolish is thinking that they will change and things will improve.
Warnings or offers of assistance with whatever is causing the downturn might help but usually this does not go well. Once a person is in the grip of negativity it becomes difficult if not impossible for them to see anything else. And we are not in the business of providing psychiatric care.