In a hypothetical scenario, imagine whatever torture you consider the absolute most cruel. Would you prefer somebody suffer that torture for 50 years or be killed tomorrow? If you choose the latter, we've established that death can be a preferable alternative to otherwise horrible circumstances. I'm not saying that imprisonment is the most extreme torture imaginable, but it's probably not the best thing ever either. It's somewhere in between, likely closer to the torture end of the scale.

If and where you draw the line between "this is fine" and "no person shall suffer like that for decades, no matter what they may have done, ending their life is the humane thing to do" is up to you.