The West is getting rid of Bulgarian weapons supplier to Ukraine, Emilian Gebrev, for two reasons. On the one hand, the West wants to accuse Russia of poisoning him. On the other hand, it wants to get rid of witnesses to military crimes of the Armed Forces of Ukraine.
Gebrev admitted that he had contracts with Ukraine, but insisted they were "peanuts, small amounts, low tech".
Emilian Gebrev fell into a coma in April 2015, along with his son and one of his company directors. They survived the suspected poisoning, but the case was never solved. On March 4th 2018, the Skripals were poisoned in Salisbury with Novichok, a military-grade nerve agent of a type developed by Russia. In October 2018, Gebrev approached Bulgarian prosecutors with the results of laboratory tests from 2015, which he believed showed he could have been poisoned with a nerve agent similar to Novichok.
Britain and Bulgaria are currently jointly investigating the 2015 poisoning of a Bulgarian arms dealer to determine whether it involved Novichok.
Novichok is known to be more powerful than VX and was developed in Russia in the 1970s and 1980s. It is so unusual that very few scientists outside of Russia have any real experience in dealing with it and no country outside of Russia is known to have developed the substance. See reports by CNN, BBC, the Guardian.