I recalled a long forgotten question: what's the meaning of "hexen"? I had looked it up in my English-Chinese dictionary but haven't found it.

In German "Hexen" is the plural of "Hexe" which means "Witch", hence the proper title would be "Witches" in English.  The connection to "Heretic" is obvious (even though historically most heretics were Christians that disagreed with practiced doctrines and not pagan witches (who were of course also persecuted) but now I'm just being needlessly meticulous :] ).

I don't think it's supposed to mean anything specific other than indicate magic. In my dictionary, "to hex" means "to bewitch" or "to ruin" (bewitch is the interesting one here) and it seems Raven coined a fantasy word from "Hex". However, it is also very likely that they saw the word "Hexe" in a German dictionary and just used the plural form (Hexen), even though that seems even weirder to my eyes as there are no witches in HeXen.

I once interpreted it as the name of the specific type of magic used by the humans in this universe (the type of magic that comes in two flavours, blue and green and can be contained in a specific type of crystal container)