(condensed from Ein Ayah, Shabbat 1:56)

Gemara: There was a student who learned a lot of Mishna and Torah and studied a lot with Torah masters, and he died at half of his lifetime. [The gemara goes on to tell that this occurred because he was particularly lenient in his personal life when it came to family purity.]

Ein Ayah: The days of a person’s life can be broken up into two parts. The first part is when he completes himself, both in the physical realm and afterward in the Torah realms of knowledge and understanding and in the realm of ethics. In the second part of his lifetime, he should begin to influence others, sharing the goodness he has developed with those younger than he, so that they too can follow the straight path.
There are people who have ruined their standing in their personal development, and they are not even worthy of receiving a complete first stage of their life. About them the pasuk says, "Men of blood and trickery will not have even half of their days" (Tehillim 55:24).
In contrast, the young scholar about whom the gemara spoke was fit to complete his allotted days from a personal perspective. His shortcoming, which made it necessary for him to die, did not have to with his personal development but on the impact that his actions could cause in the future.
His problem was in regard to the second part of his life, which has to do with preparing the next generation and the impact on young people with whom he would come into contact. He was liable to cause deterioration in those who would receive from him. The small fissure in the scholar could turn into major deficiencies in those whom he would lead. That is why he died at half his lifetime, to avoid those dangerous outcomes and to allow others to train the next generation in the correct way.