Putting an Animal Down

So today my thoughts go out to my dog Garbot (Gar-Bo) who was put down yesterday. I found out today that we had to put her down because she could no longer walk, she was anemic, and she also had some problems with her pancreas. She's about the same age I am. Now something I've been confused about for a long time is how you could put an animal down. I distinctly remember when my friend's dad had to put their dog down. My friend screamed and screamed at her dad telling him he killed the dog. This confuses me. On one hand, yes the dog clearly would've lived longer if it hadn't of been put down. The question is how long? Minutes? hours? days? So it does make me angry that my grandma put our dog down, BUT although the dog would've lived longer, it would've lived that much longer in misery. When you have so many things wrong with you to the point where you can't walk, hear, see or eat, what is the point of living? I feel like in most cases this severe, putting the dog down is the right thing to do. The dog doesn't suffer, and you know that you won't have to come home from work someday to see your dog dead on the floor. I guess there are two ways to see it. I just want to say I am so thankful for the years that Garbot did live and my other dog Mabel as well.  I'm thankful they were put down when they needed to be. It was what was best. My extra love goes out to them both. I hope this helps anyone in a tough situation! Animals cannot talk, and whichever decision you eventually make, I'm sure it's what is best. Hopefully you see both sides a little better! Thanks for reading!