Is involuntary manslaughter considered a minor sin, given that prophets are infallible regarding major sins, and Moses (peace be upon him) killed a person unintentionally?

Killing is a major sin, specifically intentional killing with aggression, and semi-intentional killing. However, Moses's (peace be upon him) killing of the Coptic man was not of this nature; rather, it falls under the category of accidental killing. Moses (peace be upon him) regretted it and sought forgiveness from his Lord, because a prophet should not kill until commanded to do so, and because prophets show compassion for things that others do not. Some have suggested that he did not kill him intentionally, but rather jabbed him, intending to repel his injustice, and because jabs and pokes usually do not kill. This incident occurred before Moses's prophethood, when he was twelve years old, or forty years old according to some.