So what were my reasons?
I should begin by explaining that this character was not originally intended to be part of the continuing cast. I had intended to kill him off in When Maidens Mourn, the first book in which he appeared. But I kinda fell for the guy, and I couldn't do it. Plus I realized what a great and useful character he was, so I let him live. But I never intended for him to live forever. Why?
The main problem is that he is too much like Sebastian. He not only looks like Sebastian, but his character is quite similar, too. For an author, that causes issues. Hopefully those issues didn't show because I worked hard to minimize the effects, but believe me, it was hard.
A second reason is that, once a cast of characters gets too large, it becomes unwieldy. Different readers have their favorites and they tend to be disappointed when Hendon or Kat (yes, some readers do like her!) or whoever doesn't get much page time. For me, plot and pacing are always trumps, so I never bring on a character simply for the sake of spending some time with him or her. But the more characters there are, the harder it becomes to work them all into the plot. The longer this series goes on, the more characters we're adding. That meant some judicial pruning was in order.
I found that in many ways this particular character's scenes were in danger of always hitting the same note. Plus, the more we--and Sebastian--got to know him, the less enigmatic he became. I wanted him to remain something of a mystery; a vaguely ominous threat.
And while this character is gone in one sense, he is not in others. His death is an important catalyst that propels the story arc of Sebastian's personal quest forward. In the next book, When Falcons Fall, Hero and Sebastian travel to Shropshire where they meet Jamie's family and learn a lot more about him, the necklace, and a certain other mysterious figure from the past. I also have a complication involving a little boy that is coming up.
Another factor that influenced my decision, although it was far from the primary one, is the realization that with a series, it becomes increasingly difficult to convince readers that any of the continuing characters are really in danger. If a reader is thinking, "Oh, she wouldn't really kill off Gibson/Hero's mother/Tom," then suspense suffers. Admit it: from now on, you're going to worry more when someone is threatened; right?
I won't deny that I miss Jamie. Because the next book, #11, When Falcons Fall, takes place away from London and thus away from everyone except Sebastian, Hero, and Simon (and Tom, Calhoun, and Claire), I didn't really notice his absence. But I will admit that it pained me when I was writing book #12. And he continues to haunt me. In fact, he haunts me so much that I've been toying with the idea of writing a contemporary mystery series set in New Orleans (although it's hard to find the time, I firmly believe it's a good idea for a series writer to occasionally do other things to keep from getting stale). And I find myself more and more inclined to make the protagonist of that series the descendant of a nefarious London tavern-keeper named Knox....