What do you call someone who believes in using other's resources instead of wasting his own purely because of the availability of it and has a motto of "why use mine when I can use others'?"
Obviously the go-to is selfish. You can look up synonyms for that pretty easily if you needed something more latinate or particular.
Going beyond that, the other people having left their resources where he could get to them was a moral hazard that caused him to act as a free rider, using shared resources without contributing. (If they aren't intended to be shared with some people, he's just a thief.) When there are too many people acting that way towards a shared resource, it's called the tragedy of the commons.
More informally, there's the word moocher: (MW)
a person who is supported by or seeks support from another without making an adequate return
unfortunately, too many people at the community potluck were moochers who didn't bring any food of their own
bloodsucker, freeloader, free rider, hanger-on, leech, parasite, sponge, spongerRelated Words dependent; deadbeat, idler; flunky (also flunkey or flunkie), henchman, lackey, satellite, stooge, sycophant, toady, yes-man; cheapskate, miser, niggard, piker, scrooge, skinflint, tightwad
A "fill in the blank" vampire... A parasite, a leech, someone above mentioned Sycophant; a personal favorite word of mine but that really doesn't apply... as Sycophants don't necessarily aim to gain anything, they just flatter to the point of it being tragic.
If you are speaking in terms of mythology; you could say succubus for female; and incubus for male, adding a "pre-noun" such as "you're a "financial incubus" (the succubus and incubus were typically only used to describe "total" consumption of the victim. They slowly over time, would either devour or destroy their victims, soul, spirit, heart, passion, lust for life, and so forth.