Something about the way the question is asked has me conflicted. The question itself implies that there is in fact a God and that God has dominion over what does and does not happen, or at least should. I don't think that's the right way of approaching such a profound inquiry. First we have to identify what God is in order to understand what it represents in relation to bad things happening. We also have to identify what exactly bad things are. I am also hesitant to use bad as a way to ask this question. I think Evil is more accurate, because people can be bad and not evil and so can a movie or food. So, with that in mind, let's delve deep.
What is God?
Well, there's no simple answer to that. There are an endless amount of opinions and beliefs when it comes to the concept of God and what it represents. But if we take a look at the representation of a God figure throughout history, we can come to relatively sound conclusion as to what God is in the grand scheme. God is the almighty, the King of Kings, the absolute top of any given culture or society. The ruler of rulers, essentially. I would argue that God is the embodiment of Order and our understanding of life in the face of Chaos - Chaos of course being the other side of the same coin. Someone might see that as Satan, Tiamat, or Seth, amongst others. Chaos to a ruler might be famine or rebellion, or things of that nature. The important thing to know about Order is that it is not inherently good or evil. Take for instance the phrase, "God giveth and God taketh away." With one hand a ruler can create peace and prosperity, but with the other he can become a tyrant. What I mean by that is a King may rule over his subjects with a velvet glove and just as easily with an iron fist. It is important to find a balance between those two ideas. Rule too soft and people will become complacent, rule too harsh and they will rebel. Order is the balance of good and evil, right and wrong, and the most appropriate course of action against Chaos. God is Order.
What is bad?
Again, there is no simple answer to that either. But to continue on with the trend above, we can establish a relatively sound sense of what constitutes as bad. To a King whom rules over one-thousand people a shortage of food might lead to famine, which might lead to five-hundred subjects dying. Most people would consider that bad for both the Kingdom and its people, assuming the King doesn't want the death of five-hundred people. Another example might be that an artist whom doesn't sell her art might become too poor to eat and thus starve to death. That would be bad for the artist and perhaps people who would enjoy her art. So what is the common factor in these two examples? In both examples, the bad aspect comes from the uncertainty what will become. So, I would argue that what constitutes as bad is the lack of Order in the face of uncertainty and unpredictability.
If there is a God, why do so many bad things happen?
See the trend? Not a simple question, but I'll give answering it a go. If we are to understand God as being Order and bad as the lack of Order in the face of Chaos (uncertainty and unpredictability), then we can begin to establish a reason why bad things happen even when there is a God. I would suggest that bad things occur with or without a God and it is through God that we find means to overcome them. Bad things happen because reality is uncertain and at any given point in space and time something can occur that throws a spanner in the works, so to speak. Bad things happen, in some sense, to challenge us and to allow growth both as individuals and as a society.
With that said, I truly enjoyed this question and would encourage readers to have a look at some of the other posts attempting to answer it:
Feel free to share your thoughts on the matter in the comment section, it would be great to see some discussion spring into existence. In any case, cheers for reading! Hopefully this has sparked some interest. Have a good one, legends.