It says before that Imae can poisoning people that having intercourse with them. For example, people always believe that MC's father, who is a human, die young because of it. So MC trying to poison the emperor. At first he thought it would be only one time process, but after so many times, nothung happening, so he lost hope and thought that maybe all of that are false and his father just got super sick and die, not because of his mother. So when he saw that, he was happy that the poison thing is true and it might actually working after all.
Ahh I thought you were talking about the last chapter, I'm sorry. My interpretation for that dialogue is that maybe MC thought that he never guess the Emperor would be care about his happiness, or him at all in general. The Emperor not getting angry at MC for broke The emperor's finger just to vent MC's anger and frustration made MC count it as 'caring for his happiness'.
Can someone explain why mc felt susdenly happen after breaking ml's finger? Was it a mistranslation cause im so confused as to why it suddenly turned to fluff