Once they become Cybermen in that episode, people lose their human bodies. They become like cyborgs (human mind and a bit of flesh in metal casing), and in order for them not to feel sad because of that, their emotions are turned off. So in order to save earth and those people who are still normal, the Doctor makes them feel their emotions again. They suddenly remember that they were human and had families and kids and normal bodies and all that, and feel horrible psychological pain ... if I remember correctly they self destruct because there was no way for them to become human again
Yeah, the idea has been used in so many films I think
There was also another episode where aliens got a bit of human blood of a certain type so they were able to control those people who had that particular blood type... and they made them stand on the edges of rooftops and then threatened the earth’s governments that unless they give up their freedom peacefully, everyone will jump
Either way seems like our freedom (or whatever bits of it we still have) is a rather vulnerable thing