Russia has the most to gain because they'll simply sell the natural gas to Asia or through another pipeline, meanwhile they want to pressure Europe into not supporting Ukraine any longer.
The Ukraine war could literally end modern Russia as we know it, more defeats and the entire current government and structure could collapse. If somehow they survive the war, years of sanctions will. The Ukraine war is an existential threat for the Putin government...
And that's not even counting the fact they love false flags and blaming others...