Millions of pets are killed (euthanized) in pet shelters every year. According to the Rachael Ray site 3 to 4 million are killed in shelters every year, in the US.
What I don’t hear people talking about is how they are killed. First, are they frightened? Do they have a sense of impending doom? I think it’s likely. Of course, some of those killed will be for health reasons, they were hit by cars, abandoned and sick, old, etc. Some will be killed just because there are too many other pets in the shelter with a better chance of getting adopted.
Secondly, how much does it cost to euthanize an unwanted pet? Then, who pays for it? Shelters run on donations, mainly. Who pays for all those unwanted pets to be "taken care of"?
Reconsider a living pet. A living Christmas tree is one thing, a living animal has more needs and isn’t just seasonal or young and smelling good forever.