Folks like this generally have some kind of bitter axe to grind. You can see the effect of the bitterness because their theology is half-baked at best, it would not pass the laugh test at any gospel based seminary, and yet they are vehement beyond measure. I doubt it’s what you speculate, but more likely that Phelps has been hurt by someone of that kind and now really wishes that God was hate.
That sounds more likely. Maybe a close friend or relative?