This assumes there is a difference under the law between reckless and negligent homicide (which does not exist everywhere). In Washington state there is a distinction between 1st and 2nd degree manslaughter, per RCW 9A.32.060 and .070. What could be called reckless homicide is when
(a) He or she recklessly causes the death of another person; or (b) He
or she intentionally and unlawfully kills an unborn quick child by
inflicting any injury upon the mother of such child.
and 'negligent homicide' would be
A person is guilty of manslaughter in the second degree when, with
criminal negligence, he or she causes the death of another person.
The distinction is laid out in RCW 9A.08.010, compare
A person is reckless or acts recklessly when he or she knows of and
disregards a substantial risk that a wrongful act may occur and his or
her disregard of such substantial risk is a gross deviation from
conduct that a reasonable person would exercise in the same situation.
A person is criminally negligent or acts with criminal negligence when
he or she fails to be aware of a substantial risk that a wrongful act
may occur and his or her failure to be aware of such substantial risk
constitutes a gross deviation from the standard of care that a
reasonable person would exercise in the same situation.
However, it is important to know what "knows" means:
(i) he or she is aware of a fact, facts, or circumstances or result
described by a statute defining an offense; or
(ii) he or she has information which would lead a reasonable person in
the same situation to believe that facts exist which facts are
described by a statute defining an offense.
The fact that X has manufactured a nutty belief that killing a person doesn't lead to death does not override the fact that a reasonable person would know that stabbing a person in the heart will in fact kill them, and is murder. You could put that in contrast to the situation where he has a belief that stabbing a victim in the butt will cure him, and he is unaware that the victim's heart is freakishly in the wrong place, near his wallet – that would be negligence (being unaware).
In general, you aren't given legal credit for having bizarre beliefs about how the universe works, unless it can be turned into an insanity defense.