In an alternate future, the cold war has never fully ended, as the soviet union never collapsed, but the two super powers have decided to work together in space, resulting in the construction of a space station the size of a city, filled with adventurers wanting to get away from earth and live amongst the stars. This station serves as the gateway to the stars, and freight is being routed through the city station to colonies around the solar system.
The grand architect of this space station is an intelligent man, backed by both U.S. and Soviet interest groups, and he is very proud of his creation. However, something happens where he comes to the conclusion that the earth must be destroyed. He sends signals to nuclear warning satellites around the globe, and the United States and Soviet Union are tricked into thinking that the other side fired first, causing a nuclear war that wipes out almost all human life on earth, while the city station is left unharmed (at least during the initial nuclear conflict, the fallout of these decisions will absolutely harm the station later on).
What would cause the designer of a joint American-Soviet space station to come to the conclusion that earth must be destroyed? His actions must be deliberate, they can't be an accident, nor can he be tricked or persuaded into doing them.