My dad tells one story about having to put down a K9 dog. This was back when they only used administered one shot with the killing dose (now many places give a strong sedative first, then the final dose).
The officer was very upset, and the dog was up on the lab table. My dad gave the dog the shot, and the officer started crying. The dog just sat there.
30 seconds went by; nothing happened. The dog just kept staring at my dad. 1 minute, 2 minutes, nothing. My dad apologized, figuring he must have gotten the doseage wrong (he hadn’t – there was more than enough in the shot to knock out a small horse). He began to prep another syringe.
Then the dog broke its gaze with my dad, looked at the officer, and the officer said “It’s okay bud. Everything’s okay.” The dog breathed out, looked back at my dad, rested his head on the table, and closed his eyes permanently.