If the devil is the master of this world would he not have all of us suffering here and now?
Although it is true that the devil is the ruler of this world he has no authority over true Christians as they are protected by Christ Himself. Jesus says in John 12:31, “Now is the judgement of this world; now the ruler of this world will be cast out.”
Those who have received salvation are in the world but they are not part of the world. Jesus also spoke to the Jews in John 8:23-24, saying, “You are from beneath; I am from above. You are of this world; I am not of this world. Therefore I said to you that you will die in your sins; for if you do not believe that I am He (Christ), you will die in your sins.” He also says, “These things I have spoken to you, that in Me you may have peace. In the world you will have tribulation; but be of good cheer, I have overcome the world,” John 16:33.
If some are doing his will why would he punish them?
The devil is a liar and master of deception. His promises cannot be trusted. He gives with one hand and takes something precious with the other hand, from those who do his will. Jesus continues to speak in John 8:44, saying, “You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it.”
So we get advice from 1 John 2:15-17:
15 Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him.
16 For all that is in the world – the lust of the flesh, the lust of the eyes, and the pride of life – is not of the Father but is of the world.
17 And the world is passing away, and the lust of it; but he who does the will of God abides forever.