You know, when Putin won last time, he actually had someone run against him when he won.
This time? One competitor was killed, and the other one was banned from participating in the election, which means that Putin basically ran unopposed, which is how dictators all over the world end up running their elections.
Trump congratulating Putin on his "win" legitimized his election.
And then, there is also the fact that Putin had poisoned an expatriate and his daughter a week before the election with some nerve agent that is only made in Russia, and the Russians are the only ones who have it. The attack happened on British soil, and Britain is one of our strongest and closest allies, yet Trump didn't mention anything about it to Putin. In addition to being told to NOT congratulate Putin on his election win, he was also told that he needed to chastise Putin for the attack in Britain, but Trump never did.