You DON'T have to stay trapped forever in that cavern with it? Basically it's the 'selfish' thing to do because you're killing a benevolent entity because of the risk you 'might' lose control of the cuthintal but if you want your character to actually have a life/future with the love interest...also depending on choices you might gain the cuthintal's respect and since apparent he's a physical being on the other side of the planet you might actually meet him one day.