As I have to learn stochastic, I'm taking another try to find out Heartstops kill chance.
For the DarkNemesis under you who dont know what Heartstop is, it is a talent that kills a unit when Exhaustion stuns it, if it is already stunned.
Exhaustion on level 6 has a 16% chance per second to stun a unit for 3 seconds and lasts 85 seconds.
Units:
Every second there is a 16% chance to cause a stun. Then there is a 0.84³= 59.3% chance in the next 3 seconds not to die or 40.7% chance to die.
So we have a 0.16*0.407= 6.5% chance to die per second.
For 83 seconds we have 0.935^83 = ~0.377% to survive one cast of Exhaustion.
But units tend to not live that long anyway, so we take a look at smaller times:
5 seconds: 28.5% kill chance
10 seconds: 49% kill chance
15 seconds: 63.5% kill chance
Heroes:
8% to get stunned, 22% to get another stun while stunned (=stunlocked).
Of course that calculation may be completly wrong, so feel free to correct me.

Also not sure if 2 or 3 stuns can happen during another stun which trigger Heartstop.