Imagine the following nightmare scenario.

You are wrongly suspected of criminal activities.

You are tortured in some way until you "confess." For the moment, it doesn't really matter whether this means your fingernails are pulled off one by one or you are "just" being kept awake and constantly iterrogated without sleep for days and days on end. (It was "just" this method, and not the more colorful forms of torture, that was used to extract many confessions in the Moscow show trials in the late 30's.) The important point, is that there is some level of coercion going on. Your interrogators are doing something to you that you want them to stop doing badly enough that you willing to "confess" to make them stop doing it.

This coerced confession is used to convict you.

Then, you're executed.

Now, officially banning torture doesn't mean that it never happens, and there are famous cases of people sentenced to death on terribly flimsy evidence, so it's not as if nothing like the nightmare scenario just sketched has ever happened, even here.

But.

If it happened, it was illegal on several counts.

Not anymore.