Write a sad ending. Poetic justice is a lie. Sometimes bad things happen to good people. Sometimes the wicked go unpunished. Sometimes love is not enough. Always, fortune is blind. To forget that would be lazy and dangerous. It makes the fortunate self-righteous and the unfortunate self-loathing. Write a sad ending, because people sometimes need the reminder; because it's one truth, and truth has a value of its own. We need stories to give us hope, but we need stories to give us truth as well, because hope without truth is a mockery and if there's no truth at all, we cannot relate.
Write a happy ending. It's true that sometimes the wicked will thrive. It's not true that _only_ the wicked will thrive. You _can_take the high road and win. Violence and lies are not your only options. It's in the nature of power to be a temptation, but if you're too weak to resist it, that's on you. Those you keep from power might not be just as bad as you once they get hold of it. To forget that would be lazy and dangerous too. It makes the ruthless complacent and the meek despondent. It blinds us to the full range of our potential. It traps us in the status quo and creates an illusion of inevitability, so awfully convenient to those who'd rather view themselves as having no choice than muster the courage to make the more difficult one.
Write an open ending. Closure is overrated. There's only one ending in real life, and only if you make the story about yourself. Happy or sad? Who could tell? There might be no higher instance to decide the meaning of it, to weigh your heart, to sum up your ledger. All there is are choices and their consequences, sacrifices, trade-offs, opportunity costs. Was it worth it? Let your readers decide. Some people like a bit of room for their own thoughts. You are an artist, not an accountant. You don't have to fix the meaning of things. You couldn't if you tried.