Now I'm fairly new to the Supernatural franchise (I just started season 3) but I ended up either accidentally reading too much wikia's or my friends have told me about things so I know a fair bit into the future of the show.
I was wondering, since there are obviously angels who have turned "evil" by way of killing other angels or just in generally doing evil things, can a demon in the show be "good"? Ruby kinda helped Sam (from what I've seen so far) but she just wanted Lucifer out.
That said, we are often shown that while demons are prone to lie, cheat, steal and kill, the more powerful demons will often put aside their personal urges for the aforementioned in search of a greater opportunity for mayhem or personal gain. The very heart of the demon philosophy is "selfish is good".
The demon Ruby is often cited as being a "good demon" but her goals were selfish and even though she was sexually active with Sam, she had no intention of helping the Winchesters. She took advantage of his connection with her to exploit him into completing her mission.
The greatest example of this is Crowley, self-proclaimed King of Hell. Crowley has repeated worked with the Winchesters in order to further his schemes and almost always promptly screws the Brothers Winchester as soon as he gets what he wants.
Crowley is a bastard willing to do almost anything to get what he wants; the epitome of demonic behavior. Only read the hidden text below after season 8...
Crowley allied with Sam and Dean in Season 5 against Lucifer to obtain the Four Horsemen's rings with them, then with Castiel to obtain the power of Purgatory and briefly Raphael in Season 6, and then with the Winchesters again in Season 7 against the Leviathans.
The moral line for Supernatural baddies is pretty clear most of the time. If you are anti-human, (use Humans for your dark magic rituals, prey on or eat Humans) you are a bad guy and if you cross the Winchesters (or any other decent Hunter) you get what's coming to you.
As an aside: the closest we come to seeing a baddie turn "relatively good" in Supernatural is the vampire Benjamin "Bennie" Lafitte. He aids the Winchesters during a time of great need. It doesn't turn out well for Bennie. Don't come back to read the rest until Season 8...
Benjamin Lafitte was a vampire, turned by the ancient one known as The Old Man, who he worshipped as a god. However, fifty years ago, Benny fell in love with a Greek woman named Andrea Kormos, deciding to abandon his maker. The Old Man discovered this and killed Benny, condemning his soul to Purgatory, the realm where all monsters go when they die.
That's actually a good question, I have pondered this myself.
It's true that there have been supposedly good demons, such as Ruby, but a genuinely good demon is rare. I am currently on season 10 and thus far Crowley has helped the Winchester brothers numerous times, even taking Dean under his wing when dean was demonised! Sure, as a demon, but I do believe that Crowley could have a little good in him, and I know this could be blamed on his human blood bender, but deep down if he was truly evil he'd have never helped the brothers, they'd probably be dead if he was bad. Additionally, there's the fact that he trusted his mother even after she was awful to him as a child.
So my final answer is yes, a good demon can exist.
