Dilan was not actually sure who Felicia was, or what her goal was when she attacked him.
However, he could tell that she wouldn't be able to attain her goal. That was something Dilan could promise!
Grasping her dagger and slowly leading it to his heart which she pierced, in the end, was Dilan's mean to show that he was neither afraid of getting injured nor less of getting killed.
Death feared him, not the other way around!
As for boiling her body from within, Dilan wanted to make her understand that he was far from weak, no matter what his Tier might be.
The quickest way to demonstrate his power was to obliterate Felicia's confidence and make her shut her foul mouth.
'I might have overdone it…whatever…'