That's an interesting distinction that I think you're getting at. Correct me if I'm wrong. Robb kills many people, ultimately for his own good/vanity, but is likable. Fray kills few people, but is unlikable. Who is worse?
I would interject the concept of relative morality. Robb doesn't understand his actions to be wrong, nor do others in his society who he interacts with. His worldview is predicated on honor, not the preservation of life. Robb is moral because he acts with honor (most of the time), Frey knowingly acts without honor and fails to meet the standards of his society.
You don't have to be faster than the shark

Walder is a sweetheart compared to all those war mongering lords.

with the way feudal society is, i would agree that most people under the banner of frey and bolton are generally good, but can't disobey their liege lord.
walder encourages his family from the start to infight and be petty.
i don't think every single bolton soldier at the battle of the bastards wanted to be there, or would consciously choose to slaughter their own fellow northerners.

Good Freys and Bolton's??? Yes, the dead ones!!! (It's a joke)

Of course there are, but they must remain silent when the leaders of their groups are crazed lunatics. Kinda like America in 2 weeks.

At this point there are no Boltons at all as far as I know. I'm sure there are good Freys, though, including Walda.

I'd say the vast majority of Freys are either good or harmless.
They aren't a group of people who hunt human beings in the forest or anything.
What were they supposed to do? Betray both their father and the king of the seven kingdoms for some POS who humiliated them, used them for a passing, and then broke the promise to them?
They didn't plan the Red Wedding but they weren't in a position to stop it. It's not how feudal society works.
In the books, I think it's very clear that what Lady Stoneheart is doing to them is flat out wrong and they are supposed to be sympathetic. Not Walder but most of the rest of them. Petty and absurd but ultimately not bad people.
It's a level of ambiguity and shades of gray the show has chosen to abandon. So, all Freys must die and everyone will celebrate their murderer.

Several good Freys, many of whom skipped the Red Wedding.
Say, do you hear that? It's the sound of the Reaper..

Foogie has been glutton free for a month! Don't call him a liar!

she was foogies favorite character. the focking glutton

Fat Walda was awesome.

And a major cinephile

"Only a man who's been burned knows what hell is truly like."

On the Bolton side:

• Michael: troubadour

Walda Frey-Bolton seemed sweet.

In the books, Roose Bolton had a trueborn son that seemed to be a good guy. (It's implied it was Ramsay that killed him)
There are a couple of Freys who seem to be good. Stevron Frey was said to be reasonable and died for Robb Stark in battle..
Robb also had a Frey squire and Robb said he was as loyal as any king could want.
Wouldst thou like the taste of butter? A pretty dress? Wouldst thou like to live deliciously?

Edmure wife and Walder's second wife seemed nice.

"Art is magic delivered from the lie of being truth"