Comment by r721
Apparently the worst mistake was contacting his former girlfriend in Russia:
>Exactly how the killers found him has not been established, though two senior Ukrainian officials said he had reached out to a former girlfriend, still in Russia, and invited her to come see him in Spain.
>“This was a grave mistake,” one of the officials said.
https://www.nytimes.com/2024/03/31/world/europe/russian-defe...
Or there's a mole in Ukrainian intelligence who sold him out