"I was so concerned with myself… So burdened by the weight of my own heart… I never considered that a man with no heart is a beast."
(Act 2, Chapter 1: 32-33)
There goes the hemlock potion...but then Argus sees Aornis's collar and the knife he used to cut her throat, which he was told was a quick and merciful way for her to die. So he's probably getting it into his head now that using that knife on himself would have the same result, even though that death wouldn't be nearly as clean for him, because it's much harder to manage than drinking poison would be.
(Act 2, Chapter 1: 32-33)