Irene's lifespan was running out; she had already given up half of it up to now.
And if she was to kill the mysterious entity before her, it would clearly require even more of her lifespan, which meant she wouldn't be able to survive.
As Karl pondered whether to take the girl's life again, he suddenly felt a shattered piece of information emerge from the depths of his soul, eventually manifesting into an understandable concept.
All "weapons" were different, and a weapon made of white light would have no effect on the mysterious entity before him.
He could only choose between "emotion" and "sensation".
To deplete nearly all of the girl's emotions or senses would be enough to create a "weapon" capable of annihilating the Sea Demon, but the choice of which to utilize had far-reaching implications.
So that was it. Karl ultimately decided to take away part of Irene's emotions and senses, cautiously avoiding the most important part of her heart.