I'm pretty sure it was a feature length movie. I saw it probably around 1984-86 (too young to remember). The only thing I do remember is there was a girl with her cat (who didn't talk or anything like that) and this old woman who turned out to be the villan. The old woman kept feeding the girl and her cat yams (or sweet potatoes... again can't remember).
The old woman eventually showed herself to be the villan and I think the girl killed the old woman by telling her that she loved her (which destroyed her evil heart). Spikes came up from the ground and killed her in an artsy avant garde kind of way. The movie was pretty noir. I don't think it was disney... seemed kind of anime if I remember correctly

Is there any chance you could be mixing things together? Because the bit with a girl and a cat being fed a lot of sweet potatoes sounds like a scene from the first Unico movie http://www.animenewsnetwork.com/encyclopedia/anime.php?id=628 But the rest of the description has nothing to do with it. The movie used to run on the Disney channel when I was a kid. That's the best I can do for you myself.

OMG! I just got done watching "Fantastic Adventures of Unico" on YouTube. You were right about the part with the Old Lady and the Girl (Katy... who coincidentally is also the cat. She gets transformed from a cat into a girl by Unico) and the sweet potatoes.

You were also right about getting 2 movies mixed up. I also watched some of "Unico in the Island of Magic" and I'm pretty sure that's the movie where the villain dies from spikes or light beams shooting through him. I am forever in your debt!

For anyone who hasn't seen these cartoons watch them on YouTube. It's amazing the ed up sh*t creators were able to get away with in cartoons in the 1980s.