The Inquisitor is a self-aligned role with aspects of both the Serial Killer and the Cult Leader. Its Night Action is simply its factional kill (like a Serial Killer); however, if for some reason the Inquisitor's victim survives the kill, the victim will instead have its alignment changed to match that of the Inquisitor. In this sense, it is a Serial Killer that has a chance to pick up allies.
The existence of an Inquisitor usually suggests that there are multiple protective abilities in play during the game so as to allow the possibility of recruitments.
As always, care should be taken with recruitment, as Inquisitor is generally able to recruit Anti-Town players and recruited players keep their former abilities. For more information, see Cult. The recruited players do not gain access to the factional kill if the Inquisitor dies.
Use and Power
Inquisitor is obviously stronger than a standard Serial Killer, and thus Inquisitor does not generally need additional buffs via role modifiers.
One side note is that if the Inquisitor dies, the rest of the cult can be wiped out by simply lynching all players who were protected over the course of the game. In fact, just seeing any Inquisitor-aligned player dying can cause this effect. Thus, it is very much to the Inquisitor's advantage to try to keep everyone in its faction alive.