But what if Illumi first tested his needles with himself? And now he still has a needle or more stuck in his brain and that's maybe one reason of many why he is so fucked up?
I think he’s fucked up because he’s the first son and was raised way more strictly and with less love than the rest of his siblings.
But imagine Illumi holding medical journals for bodily reactions to needles. Sticking them into his own arm, watching the muscle contract and change, then recording every bit in a journal. Same for appearance changing.
"July 21st 20XX
Increased the aura in the needles. When they pierce the cerebellum and the parietal lobe at the same time, the burning sensation lasts for 3.49 seconds. The result is a change in chin shape. The deeper the needle goes in the parietal lobe, the sharper the chin becomes.
End of experiment.”