That's because there's evidence that lethal injection isn't a painless process without complications. Certain medical conditions do affect the efficacy of the injection. Plus, the injection itself has three stages. If the first fails or only induces partial paralysis instead of unconsciousness, you're forcing people to either feel themselves die of asphyxiation or a heart attack due to the second and third stages. Those are considered cruel because they're not instant like hanging or electrocution (which themselves have sooo many opportunities for failure). Pike had a reason to believe that the first stage wouldn't work on her.
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Danielle Kristy did this video 3 years ago, so it ends with there being the wait for execution. However, the comments are already about the failed execution.