Time and time again, I see people suggesting using a tranquilliser in situations where animals (and often the people around them) are in danger. It is often suggested as an alternative to killing the animal.
However, there are a few reasons why using a tranquilliser may not be possible:
Lack of trained professionals on site. Aiming at a moving target is hard. Also, in many countries, dart guns are heavily regulated (for example, here’s a brief summary of the UK’s laws on dart guns).
People in the area. If the dart misses the target, it could prove fatal for anyone receiving the dose of tranquilliser.
Tranquillisers don’t work instantly. During the time in which the drug kicks in, the animal may become confused, angry and disorientated. This could place both the public and the animal in further danger.
These are general reasons why an animal cannot be tranquillised, and each situation in which lethal methods are used is unique. It’s always sad when an animal dies. However, sometimes it is unavoidable.