Google says:

While exact numbers fluctuate annually based on court rulings and state laws, the United States generally executes between 10 to 25 human inmates per year under the death penalty. In stark contrast, roughly 600,000 to 920,000 shelter pets are euthanized annually.

Despite the massive volume of veterinary procedures, the execution of humans has a catastrophically higher rate of failure and error.

Human Lethal Injection Historical Botch Rate: ~7.2% to 8%

Veterinary Euthanasia Botch Rate: Extremely Rare (< 0.1% Estimated)

https://deathpenaltyinfo.org/executions/botched-executions

https://www.kcra.com/article/lethal-injection-botched-christa-pike/73978451