Foyle's death is horrifically appropriate to what he was like as a person. It's not only hideously painful, it also attacks the traits he used to manipulate people; his good looks are ruined as his face swells, and his voice is silenced as his throat swells. He dies ugly, muffled, helpless as he liked to make others feel and in pain.
Will's plan to kill Foyle initially comes off as "get my former client to access his medical records and just hope he happens to have a deadly allergy I can use." However, given that Will's mother turns out to have the same allergy necklace as Foyle, Will probably saw and recognised it while working on Foyle's defence. So in fact it was "find out what his deadly allergy is."
