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?