So when we last saw Nikola, he had used the Chinese Prayer Stick he got in the first book to gain access to the Chinese Secret Society to find an improved anaesthetic. So in A Lust for Hate, I was waiting to see what he would do with that.
He uses it to murder people. Nikola is running a revenge operation using his specially designed carriage to murder people who have wronged others (hey Nikola, there's a woman in The Hamptons who could use your help, Emily Clarke, er Amanda Thorne or some such)
Nikola is such a minor character in this story appearing about a third of the way through to offer his services to the narrator Gilbert Pennethorne who had been swindled out of a great fortune and then again at the end to try and extort some money from Pennethorne (60 Thousand Pounds to be precise), that it almost seems a cheat to call this a Nikola novel.
There is a theory that this was an already completed novel that Boothby edited to quickly add Nikola which I cannot dispute. In this story Nikola is an outright villain killing for money with an interesting method to gain the capital for a new scheme.
I'm about to start on the next book and I hope to see Nikola back to form.