[That's not reassuring. That's not reassuring at all - but Izaya is still trying not to give in to that fear.]

Hmm, are you sure about that? If you're going to kill me anyway, it would be safer to do it right away. If you let me live, you may never get the chance.

[In fact, Izaya will make sure he never gets the chance. Not that he's trying to encourage Mikado to kill him after all, but the suggestion is worth it, when it allows him to get that subtle threat in.]