There are two groups...
…That everyone in the not-entirely-successfully hidden magical underground (called variously the Arcane World, The Occulture, or just “The society”) hates:
The Old Magic families, who, like old money families in the world without magic, concentrate as much power into as few hands as possible, hoarding wealth, magical treasures, and spellbooks.
and the Elves, who steal children to twist around and turn into more Elves, murderous predators who vaguely resemble human beings.
There’s also two groups nobody ever robs:
The Old Magic Families, because with all their power and wealth, they can afford top-flight mundane -and- magical security, so robbing one of their vaults is a death trap.
…And the elves, because they have no money, and their nests are full of murderous predators who vaguely resemble human beings, some of whom may be the children they stole last night.
Only an idiot with a death wish would try to rob either group, or so they say.
So the question becomes, what sort of person would try to rob both? On the same night, with the aim of using the stockpile of magical arms swiped from the former to break into the chambers of the latter and maybe free a few of the kids before they’re fully indoctrinated into the elf way of murder and obligate pseudo-cannibalism?
That’s us, that’s our crew. We’re exactly those idiots, and the only reason we can’t all get killed on this job is that at least two of us are technically already dead.