A huge moral Issue in LOTR
Frodo tries to say Gollum deserves Death and that he is no different from an Orc.
Note that Orcs were specifically bred for one thing Death. They were at one time Elves and Men but they were twisted until no good is left in them and they turned into their hideous form. Much like certain types of soldiers.
In some countries you have military groups or types and all they are trained to do and taught is just kill and that’s it. No prisoners, civilians get the same treatment. So they are taught to be merciless and cruel.
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So there is no good in the Orcs what so ever. Its not like you can send them to a temple turn one into a monk and he is all better. Orcs are so removed from good they are likely to destroy and kill everything in the temple and around it.
Gollum however is a different story because he was once a Hobbit so there is still good in him.
Gandolf describes Gollum as someone who is sick. This is a reference to the 1st Century Christians who saw bad ways much like a Sickness rather then a Crime. In other words it can be cured.