Kremlin desperately wanted a show trial. It was not Putin’s goodwill. He was pressured by different organisations and western politicians. The final decision was made after two agents (spies) of Russian Main Intelligence Agency were captured by Ukrainians in Donbass. They were sentenced by Ukrainian court. Russians stated those agents are no longer their employees most likely backdating their dismissal. Russians would not even let media question wives of those agents. That was the deal. President of Ukraine pardoned two agents, while president of Russia pardoned Savchenko. Without capturing the agents I think Nadia would not have been pardoned by Putin.