[Oh. This? This, Akechi can answer. He'd been prepared for this question since the first time Ren questioned how he was alive,weeks ago. Perhaps even a month by now.]

Do you think I enjoyed being left out of revenge against the one person who wronged me most? [he hisses, eyes sharpening as he meets Ren's.] After I'd spent years planning it? It wasn't by choice that I wasn't there. Do you think I'm happy with the way things turned out?

[And that, at least, is painfully true. Whatever Ren's grudge against Shido, it's nothing compared to Akechi's. There really is a part of him that will never forgive him for stealing his revenge.]

But don't you remember what I said about slowing you down?

[His eyes don't waver as he keeps them on Ren's, holding him captive as best he can.]

Dragging along an injured, half-dead enemy out of a misguided sense of righteousness would have just gotten you and all of your friends killed. As much as I disagree with your methods, it's better than nothing, which is what would have happened to Shido if you'd taken me with you.

[There. It's not a complete lie, but nor does it directly answer Ren's accusation. Instead, it allows him to fill in the blanks with his own suppositions.]