Wake up.
Trump has been laundering Russian oligarch money for years. Between this and other kompromat Putin knew he could control Trump. This is at this point somewhat speculative on my part. But if you go back to the time line linked above, the evidence is lining up...and this is *before* the Mueller report.
In addition Putin is a troglodyte of a sexist, and felt humiliated by Hillary Clinton and the way she put him down at every turn.
"In 2012 the US government approved the Magnitsky Act, a bill of sanctions against Russian entities seen to be involved in the death of Sergei Magnitsky.
...
The sanctions bill enraged Mr Putin."
"She left the State Department in 2013, but continued to criticise Mr Putin.
Over the years she has said that Mr Putin "doesn't have a soul", she called him a "tough guy with a thin skin", and she compared his annexation of Crimea in 2014 to the actions of Adolf Hitler in the 1930s.
She also supported the crippling economic sanctions slapped on Russia in the wake of that annexation."
"For the last 10 years, just about every foreign policy Mr Putin has tried to enact, from Europe to the Middle-East, Ms Clinton has vocally opposed."
http://www.abc.net.au/news/2018-05-2...lained/9783076
If Putin's first goal, though, was to destabilize American democracy and the western alliance, he couldn't have found a better partner than Trump. Trump has insulted all the major western leaders, has undermined the G7, is threatening to end NATO, is starting a trade war with all of our western partners, and is on course to recognize and approve of Putin's control over Syria and annexation of Crimea.
Finally, Putin probably recognized in Trump what we should all now know is true. By temperament Trump is an authoritarian personality type with no respect for democracy. His racism and nationalism is isolating the US. And his practice of insulting our allies is only matched by his fondness for complimenting murderous tyrannical leaders such as Putin, Xi Jinping of China, Kim Jong-un of North Korea, Rodrigo Duterte of Philippines, and so on.