Putin is dangerous because he realised that in the Internet age, you can do incredible damage to a country with social media messaging, hacking, and bribery.
The US has been using the media like this for decades, but it's always been considered a side game. Russia has made it a primary mode of attack - partly because it's vastly cheaper and more effective than spending extreme sums of money on super-weapons.
So we know that Putin has been financing the subversion of social democracy and the promotion of far-right nationalist narratives in the US and the EU through a combination of direct funding, social media troll farming, and straightforward financially advantageous diplomacy - and possibly also direct election hacking, although so far that's been harder to prove.
The UK has been almost destroyed by this. It's a shell of the country it was when the century started, and the poison of Brexit hasn't even worked its way fully through the system yet.
I expect I'll get the usual downvotes for posting this out, but the downvoters might want to address the points made here before they waste their time on that:
https://www.newlawjournal.co.uk/content/is-brexit-a-russia-b...
There's much more detail about this in a book which is being released soon.