You don't necessarily kill an animal every time that you litter, like you throw a soda can out the window and, in that instant, an animal dies, but there are many things that are killing animals...
Mainly because the animal is either unadoptable, for example, an illness/injury is too severe or expensive to cure, or the animal is aggressive towards other animals or people. However decent animals...
sadly to say they don't have the money or space to keep every animal and if it is not found a home for within a certain time period (usually 7-10 days) it will havbe to be put to sleep to make room...
Sometimes if they are too old nobody will adopt them so they put them down, so yes. Also, some shelters simply don't have the space to keep all the animals they get or they don't have the means or...