Most Zoos are very good with animals and some Zoo's will only put down an animal or kill an animal in the zoo if necessary due to a health issue etc.
But some zoos may be mistreating animals and may kill the animals purposely just because they can because I've heard of a few zoos in some other countries outside the USA that would torture and kill animals and so far they are sadly still open and mistreating animals.
The zoos that kill the most animals are Danish Zoos and EAZA has estimated that its members cull and kill between three and five thousand animals each year.
Animals in some Danish Zoos and other country zoos are killed each and every year and I wish it would stop.