Assassinations are anarchist / ultraleft tactics, we want to remove the possibility of billionaires re-becoming or regenerating into existence.
A stark example of this is during the Cuban revolution. Fidel talks about how, near the end of the civil war, they had many opportunities to assassinate the opposing military leaders, but they refused to do so. Why? Because that changes nothing about the nature of the system, and new generals would just take their place. IE we need to remove their ability to regenerate, rather than their manifestation.
Fidel said that assassinations are easy, revolutions are much more difficult, long-lasting, and worthwhile.
It's also noteworthy that the Chinese revolution took the opposite stance from the bolsheviks when it came to treatment of royalty. They proletarianized them, rather than killing them, since their base of power was eradicated.