Yeah, it would suck for him to get stuck in the Black City alone. But what of the tragedy that befell those he sealed away as well? Do they not deserve justice either? Are they to blame for madness, insanity, metal illness brought upon them through blight? I think it'd be a fitting temporary punishment. A way for him to feel personally what he had done to them and in time, find a different happiness and a different future than what he had planned. It might not be sunshine and rainbows, but it's bittersweet. With hope.
Falon'Din to Solas:
To bait fish withal, if it will feed on nothing else, it will feed on my revenge. Solas hath disgraced me and hindered me half a million, laughed at my losses, mocked at my gains, scorned my Pantheon, thwarted my bargains, cooled my friends, heated mine enemies--and what's his reason? I am an Elvhen God? Hath a God not eyes? Hath not I possessed of hands, dimensions, senses, affections, passions? Fed with the same food, hurt with the same weapons, subject to the same diseases, healed by the same means, warmed and cooled by the same summer as, you, Solas, are? If you stab us, do we not bleed? If you poison us, do we not die? And if you wrong us, shall we not revenge? Why revenge? The villainy you taught me I will execute--and it shall go hard but I will have better instruction.
Edit: I defy you to read this in the voice of Falon'Din and not get shivers.