Putin had at least two reasons to interfere in the U.S. presidential election. First, he had a grudge against Hillary Clinton (she called him a "tough guy with a thin skin" in 2014). Second, he wanted to reveal corruption in US politics and to "split off key American allies by creating the image that [other countries] couldn't depend on the U.S. to be a credible global leader anymore," according to a high-level intelligence source.
From NBC News
"It is most certainly consistent with the Putin that I have watched and used to work with when I was an ambassador and in the government," said Michael McFaul, who was ambassador to Russia from 2012 to 2014.
"He has had a vendetta against Hillary Clinton, that has been known for a long time because of what she said about his elections back in the parliamentary elections of 2011. He wants to discredit American democracy and make us weaker in terms of leading the liberal democratic order. And most certainly he likes President-elect Trump's views on Russia," McFaul added. Clinton cast doubt on the integrity of Russia's elections.