I heard that males are killed… why? Where can I find more information?

Both males and females are used for broilers, so when we buy a chicken at the supermarket we can find both a male and a female. In laying hens, on the other hand, the commercial interest is only in females, simply because only they lay eggs.
Egg breeds grow very slowly and therefore males of these breeds cannot be used as broilers (for purely economic reasons – efficiency – they need much more feed and therefore have a greater impact on the cost of production and the planet) (link to dual purpose). The exceptions are some special and niche products, such as cockerels or capons, where the slow growth rate is considered a quality.
From here on, however, chickens and laying hens follow distinctly different and particular paths.