How could God let this happen?
Actually, the question is more of a grumble against God. How can God who is almighty, all powerful, all merciful allow destruction and death to happen? God loves us and therefore God should not allow us to suffer. Because God allowed us to suffer, therefore God either doesn't love us or is not powerful enough to prevent suffering. What a flawed logic!
God allows death, hunger, war, sickness just as God allows life, abundance, peace and health. Our human bodies consider anything that cuts life or our enjoyment of life as evil. We think that because death and suffering are evil that God should not allow any of it to happen. That is were we are wrong, for suffering is not evil. Death is not evil.
For everything that dies, new life begins. Anything that is destroyed, new things get built. Such is the cycle of living. God gave us flesh and blood to experience life’s emotions, the good and the bad. But life is transient. We die, flesh and blood will decay. If we accept that, there is no reason to grumble when things happen to cause our bodies to die. God gave us a temporal life, which means He can take it away from us any time. It is also not for us to complain to God the method by which our transient lives are taken.
If you loan someone a piece of jewelry with the understanding that you can get the jewelry back any time and any way you want, will that person grumble when you collect the jewelry back? Only if that person thinks that the jewelry is his/hers to keep. Sorry, but life on earth is not for us to keep.