Why assume it's rational? The Ukraine belonged to the USSR, Russia sees itself as the legitimate successor to the Soviet Union. So the Ukraine and the Baltic states belong to it, as far as Putin sees it.
Panslavism has died in 19th century when other slavic states figured out that it is just different name for Russian empire. Then panslavism was revived after 1945 as Eastern Block. It has been an economical disaster.
moscow always called the shots in kyev and they prefer to keep that going.
also don't underestimate the black sea access and the pan-slavic, pan-orthodox "3rd rome" thing -- those also shape perception.