How do the killer bees threaten the rain forests?

They don't threaten the rain forest at all. Honey bees are naturally woodland and forest creatures.

Killer bees are more properly called Africanized honey bees. They are the offspring of a number of Tanzanian honey bee queens (apis mellifera scutellata) escaped from a research station in Brazil and mated with wild European honey bee drones (apis mellifera mellifera). They have spread as far north as the southern United States and south to the limits of tropical South America.

The only difference between Africanized honey bees and the European honey bee is that they are less docile and sting more readily, hence the name given to them by the sensationalist media.