When the victims become victimizers.

75% of the terrestrial mammal predators (over 15 kilos) are at risk of extinction. This is a serious problem for the environment and the ecosystems as the herbivores and the smaller predators are becoming too numerous. 
The great carnivores are vulnerable as they are few and scattered over large territories; they also have a low reproduction rate and they are often hunted by man, who considers them as a threat. 

In Australia, in the grazing areas the wild dogs dingo have been killed and this provoked an explosion in the number of kangaroos, which damage the grass, and of foxes, that exterminated small marsupials. 

In Alaska, the disappearence of 90% of sea otters, killed by the killer whales, is causing serious problems to the algae prairies because the sea urchins (once hunted by the sea otters) are devastating them. 

In Yellowstone there were too many deers beacuse there were no predators and this problem was solved by the return of wolves. 

Man can try to solve the excess of herbivores with hunting, but he will never be an efficient predator like the natural ones as he often kills young specimens instead of the old and sick ones. 

(Thanks to Lorenzo S. for helping me writing this post)