@ Ancien Regime- I like the German diaspora idea as the recruit pool for Werewolf or whatever they called it, thanks to Morgenthau Plan. However, wouldn't the vast majority of Germans want nothing to do with such dead-end loons?
Also, keep in mind, Germans would be watched very closely in this scenario.
You need to be somebody either so obscure or ubiquitous nobody would suspect them being up to no good due to ignorance, cultural arrogance (every immigrant wants to be 100% flag-waving Brit/Argentinian/American!), and/or apathy by the mainstream culture.
As to who'd be conducting their campaign of terror in this time period,
I select the League of Untouchables (Dalits in Hindi). They were everywhere in the British Empire, had lots of grievances with both British and Indian society and often couldn't catch a break.
They'd find sympathy with oppressed peoples around the globe and engage in vigilante justice against truly egregious offenders. YMMV as to their methods and criteria for who gets whacked. Also, the folks they're trying to "help" may not want it.
I could channel Keith Laumer, Len Deighton, or a number of other spy novelists about misguided liberation movements if you like.