Ok so how does a cancer kill its host?
It grows until it consumes so many nutrients that the other living cells don’t get enough. The host literally starves even if he eats plentifully.
The same applies for the US: The billionaires are not only hoarding wealth, but by doing so they’re crippling the economy for workers and everybody besides themselves.
Correct. But that is not what OP said. Read it again and I think you will see that OP is saying that “Billionaires are cancer” is not a figurative statement at all, but a literal one. You can disagree with them (I do, btw), but they have not misused the word “literally.”
It is a metaphorical statement rather than a simile, but both metaphors and similes are figurative, not literal.