Any of the multiple factions that we collectively call "the Deep State", which are not exclusively three letter agencies and government agents, but also the rest of the actors participating of the Corporate State status quo, like banks and big corporations, who have their own resources and networks of interest within the sort of state agents that can manage to organize something like this. Private-public partnerships for the win.

People think in very simplistic terms because they still don't grasp the nature of the Corporate State Regime in which we live in the West.

Of all the beneficiaries of an assassination and removal of Trump, the Biden clique is at the very bottom of the list. If we're focusing only on the electoral process, just to give one example of a more plausible possible candidate group, the Never-Trump Republican Neocons, who obviously have enormous networks inside the three letter agencies to this day, would benefit directly from the assassination. Should Trump be removed one way or another, they would undoubtedly deployed a plan to seize the nomination for one of their own.