Depends. Case by case basis.
The Reds who took it under orders and didn't know any better, I feel sorry for them. Especially that dying one in Sulidin near the flag. He was tricked into taking it and was able to resist. He even chose to stay infected rather than take whatever unethical choice Imshael offered him. Remember, the Reds were being controlled through the red lyrium by Corypheus.
As for the Reds who willingly took it knowing what it would do, or those who hated mages and the Chantry to begin with; I don't feel bad for killing them.
I feel fine killing all those Reds who have been corrupted to the point beyond repair. Like the codex for the Behemoth states, the person is long dead and the monster is all that remains. Killing that monster would be a mercy.
But those who are still only slightly corrupted and can be saved if taken off in time, I would have rather saved them if possible. With Denam, he had only just started, so he was able to get off it. But he did it willingly, and murdered the Knight Captain. So I put him in prison for life after interrogating him.
As for the Wardens, it's still a case by case basis. I feel bad for most of the Wardens who had to die in order for the Inquisition to get into Adamant. They were just following orders and doing what they thought was best for the world. The ones under Corypheus' control, I have mixed feelings about. They should have known blood magic and demon summoning was unethical, but by the time they realize the consequences it's too late to turn back. I kill them without any worry. And as for the Wardens who changed their mind in the end, I can forgive them. But that doesn't mean they should go unpunished. Expelling the entire order makes no sense, but I would have liked the option to punish them in some other way. Best I could do is send them to fight the darkspawn and demons in Orlais.
As for Ser Ruth, I imprisoned her. Public humiliation wasn't enough, an early Calling is just moving up the inevitable outcome, forgiveness from Andraste is not a valid option. So prison is the only thing I could choose.
I actually wish there had been a way to put Ruth to work shoveling horse crap for the Inquisition, as well as other menial jobs. Also would have been cool if her working in the stables had drawn Blackwall's eye. There are parallels between both characters and I think they both could have helped each other cope with their respective failings. I would have worked Ruth ragged until she remembered that being a Grey Warden is about service to others. Then I would have reinstated her and given her armor back.
But yeah, remorseful Wardens like Ruth get a strict punishment and then forgiveness. Wardens like Duncan get an axe to the face.