> ... at least for me it doesn’t make a huge difference compared to a 15-year sentence.

There isn't a huge difference between a death sentence versus 15 years in prison? You're kidding right?

Whether the person is in fact guilty or innocent, there is a significant difference between the two sentences, literally the difference between life or death. One irrevocably extinguishes the person's life - there is no going back. The other is a very long incarceration, cutting them off from society for that time, and offering at best a chance for correction.