Nord Stream 2 does not go through Ukraine. It is under the Baltic Sea.
Currently the Russian gas goes through Ukraine. Nordstream is solely needed to allow the Russians to bypass them and their theft and the graft that also enriches the well-connected in DC.
Nordstream is designed for Germany to get the gas directly and cut out the corrupt middlemen of Ukraine and DC.
DC is quite happy with Russian gas going to Germany as long as it goes through Ukraine. DC is angry that Nordstream allows an end run and they can abandon the Ukraine like if they want.
Also Ukraine is damming all the water into Crimea from the main canal. If Nordstream opens, Russia can do likewise with gas into Ukraine and force them to turn the water back on.