Thanks for your question.
Evil happens at night for the same reason it happens during the day because human nature is evil and corrupt. As the Lord said, in Matthew 7:21-23 "For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person.” Or as Jeremiah put it, "The heart is deceitful above all things, and desperately wicked; who can know it?” Sin and evil are in operation 7 days per week and 24 hours per day.
Why do many crimes occur at night? Because it is easier to escape detection at night. Symbolically, the night has always been a symbol for sin. In John 3:19-21 we read: "And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed. But he who does the truth comes to the light, that his deeds may be clearly seen, that they have been done in God."
I hope you have found this helpful.