I had a colleague that ended up in a mental hospital for a while after their boss bullied them for a few years and flashed colleague's major achievements to own bosses as their own. The colleague didn't like their boss for sure. I guess that might be one hint to "tyrant boss"? Would that work for you?

Yes, which is why you see words like 'toxic' bandied about.

From the OED:

1. A cruel and oppressive ruler.

1.1 A person exercising power or control in a cruel, unreasonable, or arbitrary way.

1.2 (especially in ancient Greece) a ruler who seized absolute power without legal right.