First off, there is no "spirit" in psychological theory. "Spirit" is a philosophical term denoting a non-corporeal substance emanated from a God.
Also, fear is not an impulse. Fear causes an impulse. Fear is an emotion, and an emotion can serve as a motivator.
Therefore I understand your question to ask:
Is 'fear of death' the most powerful motivator?
Since literary tropes and devices often avail themselves of psychoanalytic theories, I believe it is adequate to attempt a preliminary answer with Freud.
Freud believed that there is no native fear of death. He wrote (1908, p. 167) that the fear of death of a people increases as libido is suppressed and accumulates in the unconscious.
For Freud the libido (the drive towards life) is the strongest drive. Only in his later theories it is balanced in a healthy individual with the death drive (the drive towards dissolution, annihilation and the cessation of all activity). Therefore fear of death would have to exceed libido, to be the strongest motivator.
Psychological theory is rarely psychological in nature, but rather derives its models from contemporary high tech. In the first half of the twentieth century quantum physics gave psychology its probabilistic foundation, during the second half of the 20th century psychology was dominated by the brain-as-computer metaphor, today quantum physics and neuroimaging drive psychological theories. Since Freudian theory derives its basic model from the pneumatics of the steam engine, it is probably save to hypothesize that in Freuds theory fear of death would be as large as the suppressed libido, with its theoretical maximum being the total size of libido. If more than half of the libido were repressed, then fear of death would be greater than libido and therefore the most powerful motivator.
- The level and motivating force of Bruce Wayne's fear of death would depend on his and his society's volume of suppressed libido and the pressure generated by it.
For developmental psychologist and psychoanalyst Erik Erikson, fear of death signified the absence of integrity (1965, p. 260) and a result of the despair of having lead a life experienced as meaningless. From this theory, fear of death would most probably not serve as a motivator, rather it is a symptom of a subjectively failed life.
- The loner Bruce Wayne, leading a life of compulsive-obsessive crime hunting, might well despair at the lack of generativity and intimacy in his life, and fear death more than a beloved father of several adult children.
Leaving psychoanalysis behind, fear of death has been the subject of empirical psychology predominantly in two contexts: as an ailment of the sick (e.g. cancer patients) and old that palliative psychology attempts to alleviate; and as a cause (or correlate) of religiosity. Both fields yield no results concerning the relation between fear of death and motivation. The dearth of experimental studies on the effects of fear of death are probably due to the impossibility to induce this emotion in an ethical way.
But there is a vast amount of research into fear as a motivator, mostly from the perspective of persuasion in education (stop smoking) or advertisement (buy this or else). Motivation psychology postulates a variety of basic motivators: physiological needs such as hunger, sleep, and sex; social desires such as belonging and appreciation; changing goals and spontaneous emotions; etc. And it has been found that different personalities are motivated more strongly by different motivators. I would suspect that there are people for whom fear of death is "the strongest motivator", and others for whom it isn't, because they are more motivated by positive goals (approach motivation, which correlates with extroversion, positive emotionality, and behavioral activation) than negative goals (avoidance motivation, which correlates with neuroticism, negative emotionality, and behavioral inhibition).
- Bruce Wayne being introverted, neurotic, and emotionally negative (but active), is a good candidate for being motivated by avoidance and fear in his social life (but an aggressive desire to succeed in his career as a crime hunter). I would guess that fear of death would drive him strongly, if the Blind Prisoner managed to associate (or prime) climbing without a rope with Batman's lack of a social life (e.g. show him visions of the women that loved him but he did not love back), but that he would be rather unafraid, if the rope was withheld in the context of Batman battling crime.
- Freud, Sigmund (1908). Die "kulturelle" Sexualmoral und die moderne Nervosität. GW VII, 143-167. [english Translation: "Civilized" sexual morality and modern nervous illness. SE 9, 177-204; online available as "Civilized" sexual morality and modern nervousness, quote from p. 40]
- Erikson, Erik H. (1965 ) Childhood and Society, Harmondsworth: Penguin Books.