in Shawshank Redemption, why are we supposed to feel sorry f

Pleased to meet you too. May I ask, why exactly do you feel sorry for Red?

I agree. Particularly in the scene where he admits he wouldn't know what to do on the outside. 

Yes he is a likeable character, no doubt. I like him too, more than Andy. But I don't feel sorry for him. I don't feel sorry that he is responsible for the death of three human beings. I don't feel sorry that for that crime he was sent to prison. I don't feel sorry that he doesn't have a hard time in prison at all, in fact he is doing quite well. He doesn't suffer any sort of racial or other abuse. 

I don't hate him at all, but I don't feel sorry for him. Just like I don't feel sorry for Heywood. Just like I don't feel sorry for "Coach" (Ian McShane) in "Death Race (2008). I like them all, but don't feel sorry for them.

We usually feel sorry for people suffering. Whether it is due to abuse, illness, loss, or other reasons. I don't see Red as a character who is suffering.

But to each his own. If you feel sorry for them, that's totally fine by me.