What is a group of lions called?

A pride, and it is the only cat that lives in one. Every other cat species is essentially solitary, meeting to mate and otherwise avoiding each other; lions live in groups of related females with their cubs, plus a coalition of males who hold the group for a few years until they are displaced.

The females do most of the hunting and are usually related - daughters stay, sons are driven out at maturity. They also synchronise breeding and nurse each other's cubs, which is rare. The reason for sociality is probably competition rather than cooperation in the hunt: on open savannah full of hyenas, holding a kill is harder than making one, and a group can defend both a carcass and a territory.

The brutal part follows from the male turnover. When a new coalition takes over a pride they typically kill the cubs of the previous males, which brings the females back into breeding condition within days rather than the year or more they would otherwise spend raising young. It is not aberrant behaviour; it is the arithmetic of a male who has perhaps two or three years in charge before he is displaced in turn.