The answer is on the link in the post before yours:Why did they have to shoot it dead? It said it was cowering in fear. Could a tranq not have been used? We'll never know I guess.
However, that statement (from the zoo PR) does contradict what they were saying earlier (and subsequently removed) about the wolves posing no threat. Anyway, no doubt difficult circumstances to manage and shooting the creatures is probably the least risky and most cost effective option. Essex police do like to wheel out their marksmen when animals are involved though such as this incident a couple of years ago shooting a bullock and a couple of cows!