[brobinson]

I worked at a company in the US where HR would not let us fire an employee who was viciously attacking other employees in public. The employee in question had a medical condition covered under the ADA (Americans with Disabilities Act). Perhaps OP's case was similar.

[jonathankoren]

This sounds like HR is falling down on the job there. ADA accommodations don’t include attacking someone. That’s easily a for-cause reason.

[jawns]

Indeed. And in such a case, the greater risk, legally speaking, is a lawsuit from the employees who are victims of such attacks. The employer can't just claim, as a defense, that their hands were tied because of the ADA.