The syntax of the Hebrew is consistent with other prohibitions throughout the legal texts of the Torah. That is, the negative particle לא (lo) is followed by an imperfective verb. Although imperfective verbs in other contexts are sometimes translated as simple future tense (i.e. indicative1) verbs in English, those in the Decalogue are clearly volitional — Yahweh is expressing a prohibitive injunction, not merely a statement of fact.
Biblical Hebrew has imperative verb forms, similar to English ("Go and do your homework!"). However, unlike English, imperative verbs in Hebrew can not be negated. Instead, to express negative commands (i.e. prohibitions), imperfective verbs are used. Unfortunately for English translators, this can at times be confusing since the imperfective is also used to express simple present and future tense ideas. One simple (though not fail-safe) test for identifying volitional uses of imperfective verbs is that the verb generally comes first in its clause. This is the case throughout the Decalogue, e.g. Exodus 20:4:
לֹֽא תַעֲשֶׂה־לְךָ פֶ֣סֶל֙
You shall not make for yourself an idol....
Perhaps the most straightforward rendering in modern English would be,
Do not make for yourself an idol.
However, the construction "you shall not..." is consistent with traditional English grammar, where "shall" with second and third person verbs expresses volition (whether desire, injunction, or prohibition). This nuance is frequently neglected in modern English, and the persistence of "you shall not" in most translations of the Decalogue may in part reflect the pervasive nature of the KJV tradition. It's also the case that the use of the particle לא (lo) rather than the alternative negative particle על (al) indicates a more permanent and absolute sort of prohibition. In modern English, the use of "shall" tends to invoke an elevated register which may better accommodate the sort of nuance invoked by an absolute prohibition direct from the mouth of Yahweh.
1. If applied to the decalogue, an indicative verb would entail a translation such as "you will not kill", as a statement of fact. However, imperfective verbs are not necessarily indicative. This is territory covered by any Biblical Hebrew grammar; see, e.g., Waltke & O'Connor §31.5.