This is the 12336th most frequent Russian word.
казнить
To execute.
The verb 'казнить' here literally denotes the act of executing a penalty, typically capital punishment.
Король приказал казнить преступника.
The king ordered the criminal to be executed.
The verb 'казнить' in this sentence conveys a hypothetical scenario, discussing the possibility of execution.
Он лишь чувствовал, как его могут казнить за такое преступление.
He just felt how he could be executed for such a crime.
In this usage, 'казнить' is used metaphorically to mean being overly harsh or punishing oneself for mistakes.