Animal rights activists? Theres already plenty of radicals of them, they could be so pro-animal they're anti-human so why not side with or use non-humans. The judge could have a history of either siding against protests of labs and harassment of researchers, or is known to order the death of animals that attacked people or otherwise come before him.
Also environmentalist extremists on that same vein. (may be a bit more over-used by this point though.)
Also theres MRA's (mens rights activists) which if instead of just getting on reddit to hate women together, they actually followed through with the various threats they make, would definitely include killing judges (family court judges mostly). Backstory is easy, abusive drunken husband gets his kids taken away because hes abusive, blames the courts and all women and finds like minded individuals over the internet (so they could theoretically have some e-paper trail that can be traced). Less 'good person corrupted into extremism' though, as its mostly just starting at asshole and getting worse, but its an option.
Or off the heels of the UK thing, anti-porn activists. No-one will think of the children, and so they'll clutch pearls in one hand and a demon portal creator in the other to fight the degenerates. (judge could have ruled in favor of porn makers or something in a suit.) Also, anti video-game activist, in the same vein (Jack Thompson uses Portal 2 as a murder simulator.)