Is "Beast" a specific insult for systems?

So there's this character with DID who has committed mass murder, his alter went crazy because of stress and took it out in random people (yes, I know it's a stereotypical representation). Another character once called him a beast.

She said:

It is impossible to call that thing human. It's simply a ferocious beast. It had no outlet for its impulses, and couldn't even vent its anger without resorting to violence. That's what it was like.

People keep saying she's being ableist. I got flamed online for saying I don't think she was and I think she's referring to him being violent and a murderer when she called him a beast.

For context, she's someone who has a deep hatred for murderers and criminals in general, so she acts this harsh with every criminal. However, she IS ableist and have said other ableist things to him and another character, I'm just talking about THIS ONE line.

People have told me "Beast" is a specific insult for system, but I didn't find anything about it online. What do you guys think? Do you think she was referring to his DID? Is this a common insult for systems?