That is a very different rationale. The previous argument you gave was that
So, I was challenging the previous argumentation. As to your new rationale, I'm not optimistic about the government making society more just in all contexts. That depends on the case. But when it comes to refraining from causing massive injustice, sure that is possible. After all, the government is the activity of people. And those people can make a choice not to engage in further injustice. For example, if Paul was persecuting Christians, when he stopped doing so, he stopped committing a lot injustice. But now imagine that the government had refrained from further persecuting Christians (granting that the persecutions were happening). That would be a lot less injustice.
With respect to neighboring nations, that is another area in which much could be achieved: foreing policy. Imagine the Roman empire had stopped attacking, invading and occupying neighboring nations! Well, that would have achieved that particular goal far more effectively than just persuading a few people with little power.
As for not telling their neighbors what to do, that depends on the moment. Jesus indeed was telling people what to do on several occasions.
He wasn't always speaking in a calm, straightforward manner. For example, he assaulted the merchants at the temple. But that aside, I do agree that God would not need anything with an empire, but that is because God (i.e., omnipotent, omniscient, morally perfect) does not need anything at all, and in any event, he would not create anything remotely like our universe. But if he were to find our universe, God would intervene all the time and make it better (or just all at once). Why? Well, it is not the case that with great power comes great lack of responsibility. Imagine Spider-man sees a man raping a child for fun, and just does nothing, though he can easily overpower the rapist. Surely, Spider-Man would be failing to act in a morally proper manner. And if Spider-Man were morally perfect (or just as good as in the stories), he would intervene. So would an omnimax agent.
That aside, and assuming for the sake of the argument that I am mistaken about the above, then I would say; " what does a god need with an earthly empire?" Well, God does not
need anything from anyone or anything, but he is
motivated to act in different ways, according to his morally perfect character. So, what he may well want with the Roman empire would be to make it more just (and if needed, not an empire).
At any rate, we are not talking about God, but about Jesus. Even granting for the sake of the argument that Jesus had superhuman powers, that does not imply he was God. In fact, even assuming God created our universe, presumably he would not give profoundly immoral laws to any tribe (and if your rationale is correct, he would not give any laws to any tribe: period), so he did not give OT laws to the ancient Israelites. But Jesus believed (if the NT is correct) those were indeed God's laws for ancient Israel. So, Jesus, even if super-powered, was not God.
That makes little sense on his part. The Romans only had warriors with swords, shields, spears, arrows, knives, whereas he has a friend more powerful than Thor. Still, while Paul may not think of trying to persuade them with no help, that was due to a lack of power on his part. He may have thought about converting them with help from his superhuman friend.
On Saul's/Paul's part, I see it as a reasonable attitude (and corresponding behavior) to have as long as Jesus had no superhuman powers. On the other hand, I see it as an absurd attitude for Paul to have if Jesus did and engaged in the behaviors (including displays of power) attributed to him in the NT. Maybe it was cowardice as well, but that seems like secondary - what comes to the front is the absurdity of it all.
From Jesus's own perspective, it's not cowardice, either. It's reasonable if he had no powers. On the other hand, if he did had those amazing powers (far beyond those of Spider-Man), it's akin to the attitude of Spider-Man watching the rapist and looking the other way, only times a zillion given the much greater level of power and the number of evils Jesus was watching. It would not be cowardice, though, because it would not be out of fear that Jesus looks the other way. Rather, it would be immoral callousness, and generally utter failure to act as he ought to.
Politesse said:
More abstractly, I have seen few occassions where the accumulation of power directly led to the betterment of human lives. Tacitus would agree; using an empire to try and bring about peace will always deliver you a "Roman Peace".
That is very debatable, but I'd say not the issue. It's not about accumulating power - Jesus, assuming powers as described, already had far more than the emperor.
Rather, it's about persuading people engaging in horrible injustices - through the laws their pass, their orders to their subordinates, etc. -, to change their ways. One way to prevent massive injustice is to persuade those doing the massive injustice in question to, well, stop doing it! And that's what changing Roman law (and foreign policy too; thanks for bringing that up) is about. It's also changing their orders to others (not just general laws). For example, just as Saul/Paul stopped (assuming the accounts) persecuting Christians and that prevented injustices on his part, the same could be done in a much more effective manner by persuading the emperor and generally the Roman elite.
After all, if Saul stops, someone else will be appointed to do the injustice. But not if the emperor, governors, etc., stop doing the persecution. And that (i.e., Christian persecution) is merely one example of preventable injustice.