The Jews considered God unjust because He was holding people accountable for their own choices. There is nothing unjust about that, so why were they calling Him unjust? As Solomon said, “There’s nothing new under the sun,” and people are still calling justice harsh when it’s meted out against them. They still want a free ride, hoping to get to Heaven on the merits of someone with whom they are closely associated rather than suffering the consequences of their own poor choices. God is still the scapegoat when catastrophe happens.
The people were calling God unjust, but that didn’t make it true, any more than what people say today about God is true just because they say so.
God said He would hold people accountable for their own choices. In Malachi He says that He never changes. Do you suppose He is still holding people accountable for their choices? Is He holding you accountable for your choices? Do you welcome that, or does the prospect frighten you? Whether you find mercy or death depends on how you choose.