I don't think it's that big of a deal. Considering the issue of wiping out a species, we do that all the time here on Earth (no that doesn't make it okay, but we do it all the time nevertheless, I don't think it's humanity's greatest crime). Considering the issue of wiping out an ET species, that makes me more worried for Great Filter concerns than about how we'll look back at ourselves in history. If there's life on Mars, or if there was life on Mars, either case is bad news for us.