I can only give my opinion on this. I’m not saying I’m right and being a Christian, you might even say I’m biased- but my view is backed up by scripture nevertheless. But first of all we need to remember that this isn’t just about God. The devil is in the picture too and he is behind all the evil in the world. “The great dragon was hurled down – that ancient serpent called the devil, or Satan, who leads the whole world astray” (Rev 12 : 9) There are two forces in this world ; the force of evil (Satan) and the Holy Spirit (God) and we have free will to choose.
If God was to prevent all the evil, then He would have to be intervening constantly in everyone’s life all the time. He would have to stop this person from lying over here, another person from stealing over there, another person from cursing over there, another one from committing adultery over there, and on and on and on…. And it wouldn’t be real because we wouldn’t have free will. Then we would start complaining because we didn’t have free will. So we cant have it both ways ; if we are to have free will, then God needs to allow things to happen. But instead of preventing it – he gives us consequences instead. And I don’t just mean on judgement day (although that does apply too) I mean there are consequences here and now in this life. (see Deuteronomy 28) Here we are told of the rewards for obeying God, then we are told of the consequences of disobeying him.
So why does God allow the devil to do evil? We need to remember that the devil cannot do anything unless God allows it. For more understanding on this, I encourage you to look at the book of Job in the Old Testament. You will see that the devil wanted to harm Job but couldn’t because God had put an invisible ‘hedge’ around him to protect him, but only when God allowed the ‘hedge’ to be taken away, was the devil able to bring harm to Job. Although God allows evil, he also protects- but his protection is conditional. We need to firstly believe in him, we also need to make him number one in our lives (nothing or no-one should come before Him) and we need to be living as he told us to. For more about Gods protection I encourage you to look at Psalm 91.
And lastly although God allows evil, ultimately, no-one is going to get off with it, because all his ways are just. (see Deut 32 : 4) And if he let wrong-doers off with it, he wouldn’t be just, would he? Neither would he be just if he failed to reward the righteous. Is God in control today? My view ; it may not seem like it, but he is in 100% control – always has been and always will be. The bible says that when the second coming is close, there will be many signs (see Mat 24) One of the signs is that wickedness will increase and the devil will be allowed his last moment of glory. Could this be what is happening today?