AE: Why did you decide to kill off Ianto?Am I nuts, or did RTD just say he intended to fridge Ianto Jones and he’s proud of it?!?
RTD: ...[Ianto's dying] was maximum damage to Jack. And it had to be Jack who was damaged because he’s the sort of moral player here. He’s the one that gave away 12 children back in 1965 to these alien gods. So actually he paid the price to damage him, to make this a tale of retribution and perhaps redemption all come around to him, you have to kill his lover.
AE: So Ianto died in order for Jack to make that final decision about his grandson. To be so damaged he could do something so awful?
RTD: Yeah, that's what it took. I know because it’s a great story. What a fantastic story.
ETA: On further reflection, and after thinking back to my days writing for my college newspaper, I will say that it is possible that RTD discussed “Day Four” in terms of the character’s overall arc and that those comments did not make it into the finished article. However, as the interview currently stands, it makes me feel like I’m about to have an aneurysm, I’m so angry.