During the last decades, neither Russia nor the Soviet Union has been engaged in extrajudicial killings; neither in the territory of Russia, nor abroad.
Killing opponents of the regime was a practice familiar to the USSR, see e.g. the case of Leon Trotsky. There has also been a case of murdering a former Russian spy already - in 2006, Alexander Litvinenko was poisoned by two Russians in London. As the UK public inquiry has found, "the FSB operation to kill Mr Litvinenko was probably approved by [Nikolai] Patrushev [head of the security service in 2006] and also by President Putin”: https://www.theguardian.com/world/2016/jan/21/alexander-litvinenko-was-probably-murdered-on-personal-orders-of-putin