If Trump had done basic things like released his tax returns (to show no massive conflict of interest with Russian loans etc) or stopped praising Putin on a regular basis then it would be a lot easier to dismiss concerns about his relationship with Putin and whether they stand to benefit by Trump's election.
Russia/Putin has a history of crushing domestic political dissent (including assassinations) and exploiting xenophobia for political gain. There is NOTHING for the US or any NATO member to admire about Putin or the Russian political system.
Edit. I forgot to mention the proxy war in Syria! Russia has everything to gain by tipping the scales in favour of a politician that upsets that status quo in the US.