[watches each and every one of Eichi's movements from the corners of his vision. Akechi, on the other hand, stays even and methodical with his explanation]

It doesn't take long for one to die from a slit throat, but those few seconds are very agonizing. The victim usually chokes on their own blood, unable to breathe, unable to even speak.

I would say it was an attempt at a display of mercy. Likely from someone who didn't know how painful it would be for Soo-Won.