I think a preponderance of the evidence would suggest Russian involvement, from the incident response report, the overwhelming agreement of the intelligence community (on background, of course), to the excellent reporting by Joseph Cox of vice who found it odd a Romanian hacker couldn't actually speak Romanian. Also worth noting is 'the equation group' suffered a breach after American officials persisted in talking about Russian attribution after Putin had warned them to move on. Attribution is extremely difficult, but the balance of evidence remains clear.

If you study Vladimir Putin, you'll find this is pretty par the course. He practices what he calls 'asymmetric warfare', an interesting aside in itself - but in short, he's the most skilled propagandist alive today. He's known to have interfered with European elections and funded far-right and far-left groups of almost every shade; anything to destabilize his enemies. His interference with domestic US elections is for the same reasons: to weaken US international standing and NATO, and place someone in office who would be more compliant to his demands than HRC.