ok , actually not ok:

i play UK. the pope is UK. im at peace with all. i never exterminate nor sack when take a settlement. im allied with papal state, and relation with it at highest. best churches possible everywhere and priests at max allowed converting around.
then 2 inquisitors r coming and they start killing. in 3 turns they killed the king & 2 other family members : a total of 3 members out of 6 !!! : then 2 diplomats, even the one that negotiated the alliance with the papal state, and my best cardinal and a promising priest.
now; France was excommunicated and attacked Caen with their Heir in command ( the fellow was at 0 piety ! ), my King repelled them and released all prisioners. then the inquisitor is passing by the french excommunicated Prince and comes and kills my king which was at 5 piety and 4 chivalry !!!

How on M2TW earth is this possible ? ( in this occasion no mods at all - thus a clean game )

Imho this is game "bug", or a wrong mechanism.
if those inquisitors kill they should at last look for no piety at all !!!!
A mod is really imperative imho