İt depends on your sentence. We usually use it to emphasize the sentence. İf you remove it, the meaning of the sentence can be missing.

"ki" is used to connect two clauses with a relation and to empower the sentence. In the sentence "İşkenceyi kim ki savunup meşru kılmaya çalışıyorsa" it is only used to empower the statement. Translated as "Whoever tries to support and legitimate torture..."

İşkenceyi (her) kim ki savunup meşru kılmaya çalışıyorsa…. The translation is “Whoever favours the torture and tries to legitimate it…” “Ki” means “that” in most cases, but here in this context it is a bit different. “Her kim ki” means whoever (or it might be shortened in daily conversations as “kim ki” although I still prefer “her kim ki”, which sounds better). If you remove “ki” here, remove automatically “her” as well (İşkenceyi kim savunup meşru kılmaya çalışıyorsa….). In this case, the translation will be “Who favours the torture and tries to legitimate it…” That’s the meaning will remain the same.

You can remove it. It just emphasis the circumstance.

There must be another sentence in this sentence continue.