Thus says the LORD of hosts: “Let your hands be strong, you who in these days have been hearing these words from the mouth of the prophets, since the day that the foundation of the house of the LORD of hosts was laid, that the temple might be built. For before those days there was no wage for man or any wage for beast, neither was there any safety from the foe for him who went out or came in; for I set every man against his fellow. But now I will not deal with the remnant of this people as in the former days, says the LORD of hosts. For there shall be a sowing of peace; the vine shall yield its fruit, and the ground shall give its increase, and the heavens shall give their dew; and I will cause the remnant of this people to possess all these things. And as you have been a byword of cursing among the nations, O house of Judah and house of Israel, so will I save you and you shall be a blessing. Fear not, but let your hands be strong.”
For thus says the LORD of hosts: “As I purposed to do evil to you, when your fathers provoked me to wrath, and I did not relent, says the LORD of hosts, so again have I purposed in these days to do good to Jerusalem and to the house of Judah; fear not. These are the things that you shall do: Speak the truth to one another, render in your gates judgments that are true and make for peace, do not devise evil in your hearts against one another, and love no false oath, for all these things I hate, says the LORD.” Zech. 8:9 -17
The LORD of Hosts lays out how and why they have been suffering. He is now prepared to bless them. But there is a condition that they must no longer provoke Him to wrath. They must follow after Him in righteousness.
Let your hands be strong, is a call to finish the Temple. They had been nearly 20 years at the rebuilding, it now time to get it done. The delays of the past, would now be cleared away.
Every man was against his fellow. They had been harassed by their neighbors, the crops had not been good.[Neh. 4:1.7; Hag 2:16] They had not been able to pay for what they needed. As the prophet Daniel had said the temple was to be rebuilt, but in a troubled time [Dan 9:25]. Although Zerubbabel was governor he had no real authority, army or police. When he went to build a wall, his enemies told the Persian government that he was about to lead an insurrection. So worked stopped till they could explain to the government why they needed a wall [Neh. 6]. A wall kept the undesirables out. It is like a line of demarkation, those inside have rule and regulation, those outside have an unacceptable code of behavior. It protected the law abiding citizen from the lawless stranger. As every one who has ever read a cowboy novel, or watched a movie, or TV show, knows the first thing a newly established town does is hire a lawman. 
They were the wild west’s walls. Even if that lawman is corrupt it is still better than no lawman at all. When the territory got big enough for example they could petition for a US Marshals. Without such a man the returning people were not being honest, they were not dealing with one another in good faith. Because of their sin the LORD of Hosts was turning every man against his neighbour. The crops were poor and insufficient, which is odd because the ground had be fallow for 70+ years, it should have been quite fertile. There was no reason to hire workers to plow and harvest. The merchants were afraid to ship anything through the area because there were highway men making travel dangerous. Their economy was not working. Because for any economy to work there must be the rule of law.
The LORD of Hosts admits that it was Himself who sent all these misfortunes to the people. The Hebrew word for “evil” here, is not the same as the English word. In English “evil” means that which is completely bad, or morally evil, and often refers to Satan. But in Hebrew the word has many more shades of meaning. It probably would have been better to translate it here as “calamity”. Now rather than the evil He has visited on them, they are now promissed a new era of prosperity just as Haggai had said 2:18. The LORD of Hosts is going to sow seeds of peace and prosperity[ Hag 2:9; Mal. 3:20, 4:2]
FYI: Evil, ra’ can mean breaking up or ruin, or that which is opposed to good, it is translated in the English Bibles as— bad, wickedness, calamity, distress, adversity, grief, affliction, misery, sorrow, trouble, sore, noisome, hurt, heavy, vex, wretchedness, harm, ill, mischief.
You had become a byword and a curse among the nations. This does not mean that they were viewed as a source of bad luck or misfortune for these other peoples. Rather, the way all nations speak disrespectfully of other nations that have been enemies in wartime or have been defeated and become an insult, and were now considered as malefactors to all their neighbors. They were those crazy people who had offended their God.
And again an admonition to remain strong.
He reminds them that it was their father’s actions that had provoked the LORD of Hosts to judge them. The “I did not relent” could also be “I did not comfort”. Even though all the people had been forced marched all the way to Babylon, their wives and their little ones, God provided no comfort. He d id not relent in this judgment. He said it would be for 70 years and for 70 years it was. There was no shorten of time for them, as He said he would do in the Olivet Discourse.
Now He will purpose to do good toward them. But it is conditional. They must be truthful, their justice must be just. But hardest of all their hearts must be right. For the LORD of Hosts hates all who pronounce falsely, their fidelity.