They could not have kept him any longer. Giiraffes live in a herd containing either one alpha male and a lot of females, or in a bachelor herd. The males become sexually mature at about 2 years old, and are chased out of the breeding herd by the boss as soon as they start showing an interest in the girls. One lad at Edinburgh zoo was eighteen months old, and being kept just with an older female (not his mother) until he could be shipped out. She produced a foal.
So he would have had to leave anyway. It sounds as though there was not anyway suitable willing to take him. There are strict regulations on (good) zoos which are members of EAZA, and none of them would accept an animal they could not look after properly. I know nothing about the British one which offered, but I assume they could not do so.
European Endangered Species Program
Castration would involve anesthesia, which is dodgy with giraffes. Their blood pressure is tricky, with that long neck, and they often do not come out of the anesthetic. Even trimming their feet needs them to be knocked out, as it stressed them, they are rather prima donnas.
I don't like the public killing and dissection, it is a bit too near sensationalism for me. It should have been done before relevant observers and data recorded for the future, but not in front of the public. However, I don't feel it was cruel, once the decision had been made, to put him down instantaneously while he was happy and relaxed.
I am surprised that the meat was fed to other animals. That would not be permitted in this country, probably to prevent any breeding of youngsters just for predator food. Incidentally, a lot of day-old male chicks and specially bred mice are fed to the smaller carnivores (they must be dead first) All battery hens must be female (obviously) but the chicks which are hatched as replacemants by egg-producers are bothe female and male. What should happen to the male chicks?