If the timestamp is correct, this is a weird clip. I see you casting a spell and defeating troop #3, then on the enemy turn troops #2 and #4 die (the latter clearly from Death Mark) which causes Reflect damage to kill the hero. Is that correct?
What class were you running? Looks like … Orbweaver? Let me also guess, Talent Lv.100 with Rising Shadows equipped? That would explain why two enemy troops died simultaneously…
If you can find a setup able to reproduce this, can you record a second clip but at x1 game speed? (Just switch the speed to x1 prior to triggering whatever happens)
Typically, Reflect deals damage based on damage calculated and (in the case of a kill) not the actual Life/Armor deducted. There is also a … dispute … that certain forms of lethal damage (e.g. Archer’s 3rd trait) vs. reflect result in lethal reflected damage to the attacker, when most other forms of lethality (e.g. Devour) are capped at the victim’s total durability.