Lumian's fury did not mean he would lose his composure—to just disguise himself and sneak into the member of parliament's office, where he would find the fellow who had dared to "casually spit" and incinerate him on the spot.
It wasn't a ridiculous scheme, but without enough information, taking such a risk could easily turn into suicide.
First and foremost, Lumian had no knowledge of the number or strength of the heretics present in the member of parliament's office.
He also had no idea how many protectors Bureau 8 or the two Churches had assigned to Hugues Artois, nor did he know their abilities.
Furthermore, he lacked precise details about the target's whereabouts or situation. Even if he managed to infiltrate the office successfully, finding the target would be no easy task.
Lastly, he hadn't yet devised a plan for sneaking in and, more importantly, a plan for a safe retreat.