It's all about the circumstances, the Inquisitor will have allies and agents who will probably take notice of his death or disappearance. Hell sometimes enemy Inquisitors will try to figure out where he went in case of it's a ploy to come after them. Each Inquisitor has their own skills and web of connections, in truth it'd be quite hard but not impossible to kill one and get away with it but some are easier than others.<br/><br/>As for defying them, the Ascension book once more puts a lot of this into good context, at any one time several groups whether they be the Inquisition or other powerful factions may be looking for aid from the local Imperial Guard General, this gives him or her an excuse to not provide the forces that are requested as they are needed by Inquisitor X elsewhere... even if X just wants them to guard something far less important him and the General go way back. An Inquisitor in theory can demand anything and if they really want to push it they can get anything but they start burning bridges fast where more well connected Inquisitors might be able to get what they need far more easily.