How can a good God justify sending people to Hell?

Well, perhaps part of the conundrum here lies in the way we phrase that question. I say this because I’m not sure God “sends” anyone to Hell, so much as we choose NOT to accept His invitation to Heaven.
Let me put it this way:

if you invited me to live in your House, but I decline the invitation because I don’t want to be with you, or to live under your “roof and rules”, you can hardly be said to have send me away into the darkness outside; however, once I realize how special your house is, I may regret my decision [i.e. U2 is the house band, Rachael Ray makes all the meals, the Cirque deSoleil performs in between new Wachowski brothers films and demonstrations of the latest gadets from Sharper Image, etc…]. This regret [as well as the fact that there is gigantic fire outside, I’m being gang-raped by bikers, and am in constant threat of vampires hunting me for all eternity] is what makes up the Horrors of Hell.

I know I could have lived with you, but I chose not to and that knowledge torments me. In addition, the Devil and all his fallen angels have been sent to the same spot, and I am now forced to share space with them [hence the vampires and the bikers].

So, it’s not that God sends people to Hell, it’s that they choose to reject Him and once they realize their mistake they’ve entered Hell [both figuratively and, ultimately, literally].

C.S. Lewis hypothesized in The Great Divorce that many people would choose to continue living in Hell even if given many, many additional opportunities to come and live in Heaven because they could not bear the thought of giving up their selfish autonomy; after all, as Dante’s Devil said, “it is better to rule in Hell, than serve in Heaven.”