Lament
The Labyrinth was full of misery, regret and guilt, usually amongst the damned who'd tinkered with the LeMarchand's box. It was rare if one of Leviathan's creations ever felt it.
Even though cool and indifferent on the outside, the God's own Favoured Son had one lamentation which thrummed from deep within his own heart that still continued to beat despite his immortality as a Cenobite Prince.
His human senses would spill into his darker consciousness, especially when he thought of her...Kirsty, his Kirsty; her strong will and determination was the reason for his hesitance in taking her soul, of why he was weakened.
Kirsty showing him one simple vintage photograph had reminded him of the good man he once was a lifetime ago, and with a flood of old human memories came also a flood of pain, and the regret of his actions as this Hellish being he'd been transformed into. How very nearly close he'd been to taking Kirsty's soul apart piece by piece, and how very glad he was that he chose to let her go.
Since this encounter which had ultimately resulted in his unforeseen but temporary death, the Prince was resurrected with a whole new outlook, a realisation. Kirsty is, and never was, not just any old human fodder; she is interesting, exciting, a worthwhile opponent, and he wanted to show her more than just simple pain...oh much more.
And he won't waste their next encounter.