There are several dozen occurrences of that term in the NT, and looking over them for a few minutes the only ones I can find that might arguably be referring to impersonal power are none of them. (Verrrry disputably as part of that group of spiritual opposition to Christ which Paul occasionally mentions, but even then the analogy would be from human authorities, and Paul sometimes treats the group as being personal powers in rebellion to and even reconciling to Christ, becoming “the head of every {exousia}” for example.) The term is even used for authority given to the apostles in the Matt 10 scene which contains the Matthean parallel of this saying.
So unless this is supposed to be the one and only example otherwise in the NT, or one of a super-minority example, I’m inclined to go with the majority meaning which is evidenced everywhere else. Unless there are strong contextual reasons to think otherwise.
Unless a personal ability reference like {exousia} is used. Which it is.
For what it’s worth, the Matt 10 version uses a generic term for capability there which could be impersonal. Also for what it’s worth, the pronouns could indeed be translated impersonally in either Gospel.
(Also, if you’re going to translated it impersonally, don’t forget the plural, found in both Matt and Luke versions: “Do not fear those things which only kill the body, but beyond that can do noting more.”)
The local preceding context doesn’t seem to bear out the threat being from an impersonal power however in either place. In Matt 10, Christ just got finished encouraging the apostles not to fear persecution and death from personal authorities: if they call the master Beelzeboul, how much rather those of his household, etc.! (Notably the Pharisees had done just that in the incident of the sin against the Holy Spirit, which I grant was certainly a case of their flagrant hypocrisy.) Christ follows up with a warning that He will (personally) disavow those in front of the Father who disavow Him before people. Warnings of personal distress from family persecution are included before end of that address and that chapter.
So the context of Matt 10 before and after verse 28 are repeatedly and strongly warning that those persons who can kill the body are going to do so, but keep on going and don’t fear them. I grant it’s likely Matthew ported the saying of warning and consolation from the Luke 12 address (which is a different scene) back here for topical convenience, but he dropped it into a context of personal threat to the body and encouragement not to fear those people who can harm the body but that’s all.
I’ll also grant that Luke (in my harmonization judgment) has a tendency to cluster teaching portions out of chronological order, and that this was most likely part of the teaching on the road during the final approach from Jericho to Jerusalem before Passover (which Luke spreads out as a central saying source throughout the central portion of his Gospel), whereas the dinner with the Pharisees back in chapter 11 most likely happened much earlier, maybe even more than a year earlier. But Luke has at least put them in close proximity for topical purposes, and while again I’ll grant that Luke has almost certainly spiced up the dispute with the lawyers and Pharisees at that dinner with sayings from the Greater Condemnation denouncement vs the Pharisees at the Temple on Tuesday or Wednesday of Holy Week (a scene he doesn’t otherwise include in his Gospel, so this is as good a place as any to thematically include them) nevertheless the point is that Jesus has thrown down hard against the Pharisees just recently in the narrative and so (11:53) the scribes and the Pharisees are beginning to hem Him in dreadfully and to be quizzing Him concerning more things, ambushing Him, seeking to pounce on something out of His mouth in order to accuse Him. And that’s personal persecution with intent to get the crowds in favor of killing Him.
That’s the context of the “leaven (sin) of the Pharisees which is hypocrisy”: personal persecution by religious authorities to the death. Be not afraid of the ones, therefore, that are killing the body and after this do not have anything more excessive they can do; be afraid of the one that after killing has authority to be casting into Gehenna.
Local context afterward includes a judgment warning (just like in GosMatt) that those who disavow Christ, which (like GosMatt) uses a term involving personal renunciation of Christ to other persons (disavowed before men, or avowed before men), shall be disavowed by Christ before the Father (or avowed).
(Also there’s another callback to the sin of hypocrisy of the Pharisees at the incident of the sin against the Holy Spirit, which Luke provides direct reference to here 12:10. He hadn’t included that point when relating the incident earlier, unlike Mark and Matt.)
What follows and ends this pericope? A warning that the disciples will be persecuted by human authorities, but encouragement that the Holy Spirit will help them defend themselves.
So again, in somewhat similar and somewhat different ways (including thematic connection to prominent Pharisee hypocrisy scenes in GosMatt which GosLuke happens not to otherwise report), the situational context locally before and after Luke 12:4-5, involves persecution by personal authorities. For whatever reason, he’s placed it into a context of personal threat to the body and encouragement not to fear those people who can harm the body but that’s all.
Possibly Jesus had in mind Isaiah 8:12-13, where YHWH is encouraging people not to fear the coming Assyrian punishment, even though it was going to result in death, but to fear and dread YHWH the holy ADNY of armies (Who was the one authoritatively sending the evildoers to destroy both houses of Israel). I don’t know of a verbal connection to those verses here, but thematically they not only fit the notion of fearing God instead of human persecutors, even if they persecute you to death, but also could be a preteristic connection if someone wanted to tease that out I guess.
I should think neutering this passage of its power to encourage people to stand up for Christ against those who might kill you for doing so, since Christ will judge against you if you betray Him, reduces its power significantly; and more to the point, ignores many of the surrounding contexts. It’s explicitly a warning to people currently following Christ, and there are ways to ignore the surrounding contexts which would admittedly lead people already Christian to never mind about it. But I’m against those interpretations, too, for exactly the same reasons as above.
And I do agree that sin, especially hypocrisy, destroys our soul – and often our body, too! (Even though sin doesn’t have any real {exousia} to do so. ) I certainly don’t mean to deny that, even if I don’t think the immediate grammar and local contexts (fore and aft, on either Gospel account of this saying) add up to that.