"Why can't you live for yourself? Even if it's just for a few days!" Chrollo said to Kurapika.
Indeed, there was really no need for him to kill Chrollo. The latter's heart was strangled by the chains due to the forced use of telekinesis, and his death was only a matter of time.
What's more, his consistent principle is not to kill those who are unable to resist.
After a few days of living together, the two people discovered that the other person was actually very similar to themselves, and they couldn't help but develop feelings for each other.
In fact, Xiaoku is quite sympathetic to Chrollo's life experience. So, since Chrollo is about to die, why are they still discussing about the brigade and the clan? It's better to cherish the remaining days together.