My son cannot do it for anything that he has sentimental feelings for. He calls me... and it is getting harder as I get older... yet I always think that it is kinder than having them suffer. I cried when I put down my first "beef cow"... I raised her from a baby calf.... a char/hol cross we figured.. and she was near 15-16 when she went down with a 5 month calf on her... I sat and talked to her for a bit after I found her, and then got a gun and did it... and cried.
Neighbor asked me to put down an old horse of theirs, that was down, and it was harder on me than them; they just wanted to save a vet cost I found out later...
I was lucky, my old mare was mid 20's, and she just passed away one night... got her when I was 15, my first "owned horse"..... at 1 1/2 years old.... had her until I was nearly 40......
Hardest one was my son's aussie... she was real good at staying out of the way when he was mowing...often rode in the tractor and then would want out to stretch her legs and go exploring, but this time she got in some thick stuff... he didn't see her, and the next round he saw her just sorta sitting there... and he stopped and it had cut her up bad... and he ran across the road to my house, and asked me to come quick and to bring the gun... and when I saw her there was no way she would have made it to the emergency vet even if I thought they could save her...he was beside himself because he had always been watchful of her and he said she had gone along the edge of the woods just a minute before that where he saw her.... and he couldn't do it, so I had to..... never ever want to have to be in that position again... but it had to be done or watch her bleed to death..... He was beside himself and still misses her to this day.