All men are presumed innocent until proven. Ei incumbit probatio qui dicit, non qui negat (the burden of proof is on the one who declares, not on one who denies).
A prominent Putin critic is poisoned by Russian agents, using a rare poison that could only have come from a handful of places (including, for instance, Russian nuclear reactors), and the Russian agents flee to Russia, where the Russian government protects them (and where one of them ends up serving in the Duma).
And you don’t see pretty strong circumstantial evidence of Putin’s involvement?
The question has nothing to do with a court of law. People investigating the case believe Putin was involved.
What investigation tells Trump otherwise?