Soviet Motives for Gareth’s Murder?
1.Japanese implication (& embarrassment) with Gareth’s 1935 murder by Mueller’s articles effectively resulted in no further territorial expansion until the ‘Rape of Nanking’ in 1937, arguably the start of WWII.
•Allowing NKVD Wostwag not only to continue to covertly supply weapons to [Chinese Communist leader] Mao on his ‘Long March’, but also ease Soviet fears of  a war with Japan.
2.Why would ‘Shrewd’ Purpiss of Wostwag afford a free lift to Gareth; a known enemy of the Bolsheviks?
•Gareth was a ‘loose cannon’; his liquidation by NKVD operatives would certainly have pleased (and probably countenanced by) former Chekist, Foreign Commissar Litvinov; previously & personally incensed by Gareth’s affront to expose the Holodomor, during delicate negotiations of diplomatic recognition with USA in 1933.
Gareth with Pres. Hoover, White House lawn, 1931.