If this is true I don't understand why?

I've been told Christian EXTREMISTS not regular ones like myself have turned up to gay people's funerals shouting they will go to hell... that to me is very un-Christian and contradicts the bible entirely. Whether or not you agree being gay right or wrong, once they have passed, for a start that seems very arrogant to assume you know when only God should judge what happens next and secondly Christianity is about loving and respecting one another, and if you believe its wrong forgiving for mistakes especially at this stage.... why would they do that?