You can of course be ruthless – having or showing no compassion or pity for others. I don’t recommend it, but it’s possible. Logically then, you can be ruthful – being full of compassion or pity of others. Normally when we add the prefix -less to a noun, we can add -ful as well to mean the opposite: careful/careless, hopeful/hopeless, and so on.
We don’t say ruthful though, do we? It sounds weird. It was used long ago though. Ruthless and ruthful are both derived from the now-archaic noun ruth, meaning sorrow or pity for others. Sometime around the 17th century though, ruthful began to fall out of favour. We can say rueful, which shares the same basic etymology, but has a different meaning, being full of regret, rather than sorrow for others. But why would we lose ruthful?
I think it’s mainly because being ruthful simply isn’t as interesting as being ruthless. Having compassion for your fellow man is great, but doesn’t make for great excitement. Ruthlessness though, shocks us by transgressing social norms.
If you think about it, there are other cases of us using -less and not -ful, for similar reasons. We talk about the homeless, but not the homeful, because what would we have to say about them? We take having a home, or at least a place to stay, for granted, so we don’t need to have a word to describe that state.
And perhaps this suggests a more positive reason for the fact that we don’t use ruthful anymore. Maybe we just take it for granted that most people have a basic empathy for others, and it’s therefore not necessary to have an adjective to describe this. Despite all the bad things we hear about on the news, perhaps the majority of us, despite our flaws, are fundamentally decent. Fundamentally ruthful. Makes you hopeful for the future of mankind, doesn’t it?
English-Language Thoughts 
Or (with apologies to Stephen Colbert), you could display your ruthiness.
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I’ve just crunched some numbers from a very extensive list of word frequencies and Google Ngrams. The only –less word which does not have a corresponding -ful word is endless – there is no endful.
Ruthful is surprising relatively common. The ratio of -ful to -less words ranges from meaningful being 3.5 times relatively more common than meaningless and pitiful and colo(u)rful being 4 times as more common than pitiless and colo(u)rless, through useful, careful, painful, powerful, cheerful, faithful, doubtful and helpful being 10 to 100 times more common (I’m surprised that ‘helpless’ is so comparatively rare), dreadful and peaceful being 1,000 to 10,000 times more common and delightful, grateful (I suspect that grateless refers to fireplaces and not to gratitude), successful, wonderful, awful (v awless and aweless), and beautiful being 10,000 to 120,000 more common (there are many ways to say varieties of beautiless).
On the other hand, ruthless is 1,800 times more common than ruthful (relatively more common than dreadful v dreadless) and homeless is 12,000 more common than homeful (relatively more common than delightful v delightless).
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More thoughts, more systematically: https://neverpureandrarelysimple.wordpress.com/2017/09/15/ful-and-less/
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How could I not have known the word ‘ruthful’ with a name like mine? That’s shameful… 😦
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Maybe that just goes to show how much we take being full of ruth for granted, that even a Ruth like yourself doesn’t think of the word 😊.
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🙂
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lol ruthfulness does make me pretty hopeful for the future 🙌🏻 so interesting!
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[…] read is English Language Thoughts, by Niall O’Donnell, an ESL teacher in Ireland. Yesterday, he posted about the (non-)word ruthful (the opposite of ruthless). He says “We don’t say ruthful though, […]
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Oh! Niall, you are heartful!
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Thanks!
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[…] while back I wondered, considering one can be ruthless, can one also therefore be ruthful. Toaday I considered a similar case: can you be […]
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[…] needed then, and what better than -y? As a suffix to form an adjective, -y could be used to mean full of or characterised by, which we can see in words like juicy or […]
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