NHF – new heroes for Heroes V 3.1.

There is also a negative aspect to the deadly strike: when dealing normal damage, Death Knights will destroy a much larger number of creatures...

As far as I remember, in that case, the ability will definitely not trigger. That is, the strike is always beneficial.
But there is also a downside: the rounding is done downwards, meaning fewer than 50% of the creatures are killed. For example, out of 3 dragons, the ability will only kill 1, and against 1 dragon, the ability will not trigger at all.