The definition of veganism, as decided by the group who thought up the word:
"A philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of humans, animals and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals."
So no, there is no justifiable way to deliberately kill something that does not want to die. There are however some more excusable exceptions such as to prevent suffering, or for survival or defence - survival being highly unlikely in the western world and defence being more about parasites and infection.
We live in a non-vegan world however, which means there will always be some suffering caused by our existence, and as such we can only do so much as is actually possible (rather than convenient).