why do ladybugs, flys, spiders etc come inside to die?

Answer 1

they don't come in to die, they come in to seek shelter from the elements, and what better way to do that then to enter into a climate controlled home with lots of potential food. Many of the critters that come in during the autumn months are there to over winter. many spiders like sac spiders, jumping spiders, and ground spiders will come indoors to over winter, al the same time, they will take advantage of any other spiders or insects that may have had the same idea, and will make a meal of them.
Ladybugs, the Multicoloured Asian ladybug in particular, is an aggregate diapausing insect. In other words, they will overwinter in large masses. Our native ladybugs will over winter either as eggs or pupae, but these Asian varieties prefer to overwinter as adults, this is why you get large aggregations in window sills and outdoor sheds.
Flies will also overwinter in homes, or will come in at any time of the year to steal a meal. Flies are very good at finding the food they like and unfortunately, our homes are full of goodies for them.

I hope this helps!