The humor arises from the combination of two very different movies, 'Home Alone' and 'The Purge', where the former is a family-friendly Christmas comedy about a young boy defending his house from burglars, while the latter is a horror film about a dystopian society with an annual event where all crimes are legal for one night. The idea of a grown-up Kevin from 'Home Alone' setting up traps in the context of 'The Purge' creates an amusing and absurd crossover scenario.