I decided to get as much information as I could on this topic.
I found out that the Zoo is killing a giraffe because it will affect breeding. It will affect it because the zoo let's animals breed naturally.
Why is this a problem?
If they only breed naturally, this giraffe might breed with one of its relatives.
Copenhagen Zoo doesn't do inbreeding. So a simple resolution would be to send this giraffe to another zoo, since there were offers.
Well, there are two reasons this won't work. One of the zoos who offered already has Marius relative. So if he was sent there, that wouldn't work.
Also, the zoos company doesn't want their zoos to sell, or giveaway their animals. Another thing I thought was surprising was that many people agreed with the zoo, and that many places do this. How come I never knew!