Russell T Davies has always said that Ianto had to die in order to "push" Jack into killing Stephen, something he "wouldn't have done" if Ianto had been alive. However, killing Ianto was the first thing he decided about the plot, whereas killing Stephen was the very last thing he decided - indicating that saying "Ianto's death = Stephen's death" is like saying "I saw a pigeon = I decided to eat a BLT sandwich". A does not equal B in any plausible way.

Basically Ianto was killed for no reason that other than it could happen - as shown by the klunkiness of the death scene. It was completely contrived - Ianto had no purpose in being there other than it would lead to his death. The plot was stretched to accomodate the death, rather than the death coming naturally out of the storyline.

Ah well. *Goes to plant face into a wall*.