Hi Fuzzy, you are so caring!!
And I've got to completely agree with being there for the one they're targeting. But they are probably feeling quite vulnerable from it all as well, so if maybe you could make them feel a little better about themselves, a little more confident, a little more empowered?? It might help them subtly confront the bullying, subtly challenge some of the things they are doing.
But is there any chance that you could talk to some other co-workers as well?? It might be very hard/impossible for any one of you to deal with this bully alone, but maybe if a few of you could come together and help each other??
And the blaming others bit, well I can see how it might just be their word against someone else's??? but maybe if you're documenting things/getting evidence/covering yourselves or just showing a pattern that might help in letting their superior (?) know???
As for understanding...........well it could be lots of things.....like them feeling powerless, wanting to feel in control, taking things out on others, having been bullied themselves, being jealous of their targets.........but as you know anyway Fuzzy that doesn't justify their behavior/won't ever make it acceptable...........so instead maybe just try all you can do to not make things too easy for this person, to look after/protect yourself and their target from them.

Alison