They can be simply locked away for good. That'd be precaution enough. Killing them is more about punishment, satisfaction and revenge.

Yes, that's why I'm against unfair treatment or torture regardless of the crime committed, but capital punishment with severe criminals is likely necessary. I'm not saying I've the answers, just that that's how I view this topic with the resources we have today.
If we lock them away we're slowly killing them anyway so why is that more humane exactly? It's not an argument to pretend to keep them alive only because they're alive when in fact, they're just existing in some deep hole. That sounds like to me that we're afraid of doing something that is hard but necessary to achieve an important goal.

I'm not saying, I'm against punishment and you're right pointing out the severe condition of a life sentence. This is indeed a difficult topic and in my eyes there's no clear pro and contra.

It sounds like that, but it isn't. I neither see the necessity nor the important goal. But maybe rather in showing respect for life. Children growing up in a society, where nobody should be killed, even by the state, will have a certain ethos, I find appealing.

I agree.

The important goal is safe guarding lives without condoning to inhumane behaviour such as torture or throwing a criminal into a small box he may never come out of, only giving him nutrition to sustain his or her static life.

As for showing life respect, I find safe guarding potential lives a serial killer may take is respect enough. There's also this argument against the criminals who take lives that they have evolved to something different than humans e.g they have different laws (sort of how we deal with terrorists) but that one specifically I don't personally agree with but it's still there and I find a part of it important.

But they will never be at large, anyway.

.. and I've to disagree completely. Denying those specific criminals humanity is very dangerous. If you deny humanity to anyone, it becomes possible, which I decline, like a lot of constitutions as well as human rights do.

I know, after all this mambo jumbo about values, society and so on, this stale feeling stays, that they still deserve it. I have no answer to that.