Ah, that is. Not a question that is easily answered. Not without a lot of explaining something that Ianto's not all that certain of himself. Jack didn't exactly stick around to explain his immortality very well, before he'd run off with the Doctor. Though Ianto had gotten the gist of everything he'd told Gwen so far from Gwen herself, at least.

"Not the worst idea," he replies, raising an eyebrow at John, "though I'm not sure that wouldn't have blown up the entire city in the process. No, he..." Ianto dances delicately around the reply. "Abaddon fed on life, yeah? He seeped the life out of things. So I guess you could say we gave him something we had on hand that. He wasn't able to feed off of, and he couldn't handle the result."

A delicate way of saying he sucked so much life out of Jack that he died and Jack himself didn't come back for three full days. But. That is neither here nor there.