In Matthew 10.28, the Greek texts reads:
Καὶ μὴ φοβεῖσθε ἀπὸ τῶν ἀποκτεννόντων τὸ σῶμα, τὴν δὲ ψυχὴν μὴ δυναμένων ἀποκτεῖναι· φοβεῖσθε δὲ μᾶλλον τὸν δυνάμενον καὶ ψυχὴν καὶ σῶμα ἀπολέσαι ἐν γεέννῃ.
Many languages, of course, do not translate perfectly into each other. Rules of syntax and grammar do not always have a corresponding equivalent in another language.
Biblical Hebrew, for example, uses the word 'et to designate the direct object of a verb; there is no equivalent to this in English, so 'et is often unseen in English translations of the Hebrew scriptures.
The definite article ('the') in ancient Greek functions differently than it would in English. It can, in fact, function in place of a noun in certain circumstances, such as when the identity of the subject of a verb is assumed by context.
Additionally, as with all nouns, verbs, and adjectives in ancient Greek, the article declines based on the gender, number, and case of the noun it specifies. This is reinforced by the declension of the verb the article accompanies.
τῶν and τὸν are the same Greek word, but they have been declined with different number and case.
τῶν is the masculine, plural, genitive. This means 'those who' is the correct translation. A general crowd (men and women) is being specified.
τὸν is the masculine, singular, accusative. This means 'him' is the correct translation. A specific masculine individual is being specified (namely, God).
So translating the text as:
Do not fear those who kill the body but cannot kill the soul; rather fear those who can destroy both soul and body in hell.
Do not fear him who kills the body but cannot kill the soul; rather fear him who can destroy both soul and body in hell.
are both incorrect, because they do not match the Greek text.
The first instance must be plural, and the second must be singular masculine, such as done here in the NRSV:
'Do not fear those who kill the body but cannot kill the soul; rather fear him who can destroy both soul and body in hell.'