I find it hard to wish pain and / or suffering on anybody, even mass murderers. If "justice" means inflicting suffering on anybody, even those who've caused suffering themselves, then all I can say is that I don't believe in it.
Having said that... when this guy dies, I will be quietly relieved.
Fred Phelps' son is an atheist and a personal hero of mine. He's talked at length about his family and about the relationship he used to have with his father before he was "cast out". Not in a hateful way, but rather in a more regretful tone.
I'm the only non-Christian in a family of Christians myself. I think the level of "faith" differs from person to person in our family, and we're all very tolerant of each others' views. I say this because they, and many other moderate Christians, were as unhappy with Phelps as his many, many victims were. Phelps is the reason that many people who share my views "look down" upon many people who share those of my more religious family members.
I don't have faith in Faith. I don't think there is any virtue in blindly accepting a point of view just because "everyone else thinks it's right". (Everyone else thought the world was flat at one point, too.) I also think it's far too easily manipulated by people who have an agenda. Go to war? God wants us to! I don't hate gays, God does! Etc.
However, if there HAS to be faith in the world, I'd much rather it be in the hands of the present progressive Pope than the last one, and I'm really really happy that there's one less person using Christianity as a reason to preach messages of hate and division, when to so many less vocal people out there it means the exact opposite of that.