Evil "Dutch"

This was the first of the Mister (Parallel) Universe episodes. 

Dutch and Dillon find themselves back at headquarters when they are shown a new super computer which is able to access parallel worlds with totally different histories. This world has Dutch becoming a villain after the death of his beloved dog. The episode ends with Dillon fighting him on the roof of the White House before he plummets to his death.

The first 5 "Mister (parallel) Universe" takes place at HQ with Dutch and Dillon watching different worlds on the computer. After season 5, the writers simply did away with the whole computer framing device and just the entire episode taking place in a alternative world.