I've also heard people here talk about automatic transmissions having a stall speed, and implying that a bad torque converter would cause an engine with an automatic to stall if the engine was below the stall speed.
That is not what "stall speed" means.
A torque converter's "stall speed" is the engine RPM at which the converter's normal internal function causes it to act like a physical coupler, rather than a fluid one.
To do a stall test you put the trans in gear, hold the brakes as hard as you can and start applying the gas. The "stall speed" is when the engine RPM where the torque converter's application of force overcomes the brakes ability to hold, because even though the TCC clutch isn't engaged the converter has effectively locked up or "stalled".
It has nothing to do with killing the engine.
Now a stuck torque converter clutch, staying engaged when it shouldn't can make the engine stall, but that is different terminology than what you were using there.