What would be the motive behind the hypothetical killing of Gazprom oligarchs? [closed]

In recent days a number of Russian oligarchs have died. The official reason seems to be suicide or accident.

However there is speculation that these are not a coincidence but murders, and European police is not discarding it as a possibility.

My question is, why could be the motive behind this? What would Putin or any another entity have to gain from this?