[If Mello can be a cold thing, perhaps L would welcome a reprieve from the scorching, unrelenting heat. That's what he'd had, when Mello wasn't here, and so... there might be some bleak truth to what the younger man accuses him of. If he disappeared, it would certainly be simpler; if he vanished, the status quo L's come to covet could continue unimpeded and unquestioned.

It's an evil thing to wish for. L doesn't, actively; he doesn't believe he could, even now, even when Mello makes a point of mentioning what L hasn't.

So, he's offended. Predictably. It'll only be leverage if L allows it to be, and he does have no small measure of power here. He's reminded of it starkly as Mello names his terms, sets them between them on the table like the two cups of tea.]

You... want to know why I didn't choose.

[As if that could possibly ever be it. Mello, freshly arrived, would want to know, wouldn't he?]

Two reasons. The first was that I wasn't planning to die, the second was that the two of you weren't ready.

[Yes, he said "the two of you."]

The fact that you "almost murdered N" rather confirms that you remained unready for years to come.