Zookeepers take a pre-emptive strike against the reproductive interests of male lions, at least this way the cub was killed humanely, but can't have been a nice day at work for people who no doubt love the animals in their care.
ps Lions provide a textbook example of adaptive infanticide and the brutality of natural selection. When a new male or males take control of a pride (by usurping the previous male(s)), they kill any infants, in order to stimulate the females into reproduction. This way the pride, instead of rearing cubs that the male is not related to, looks after cubs of the new male, who is the only one to benefit.
According to zookeepers, the male cub "would have been killed by the new male lion as soon as he got the chance."