I don't watch Being Human so I can't comment on it, but I completely agree that HL canon did a much better job (eta: than fandom, I mean!) wrt Cassandra having a real and legitimate anger with/hatred of Methos and a need for justice. I liked Cassandra enough in 5x01 that I didn't immediately side with Methos in the Horsemen arc. I also came across Parda's HL fic early on, which helped keep me firmly in the "Cassandra is not crazy, Methos really was a bad guy" camp, even though a lot of DM/M slash had random crazy Cassandra who needed to be put down like a dog. (Ugh.)

Thankfully I was never in HL fandom proper, so didn't know there was such virulent Cassandra-hate until much later. It's only much later, after I got more clued in wrt rape apologia, that I realised what Joe did when he instinctively supported his buddy Methos and suggested Cassandra was a liar. Bad Joe, no biscuit. Duncan, I love you even more.

ETA: I too wish the show had let Cassandra be better with a sword. I like imagining she and Rebecca sparring, way back in the ancient days, and Rebecca nagging her to practice more. *g*