[Den]

The Murderer: Lady Scrimshaw

How they did it: She slipped the laudanum into Lord Long-Name's meal when her servant was working on something else, she slipped into the service halls and caught Giselle unawares, and then used the service halls to come up from behind Isabella.

Why: She and Tavish are in cahoots and stand to gain a fair bit of business from unregulated trade and keeping the lower class oppressed.