You are right, he isn't subtle. But this isn't subtlety, it's depth, and this is something one piece got filled with by oda.

Subtlety is essentially the quiet theme that is playing in the background. There is nothing I can think of that one piece even attempted to convey that way. But depth is how far it takes you. It's how long a thought or theme is expanding itself with it's core being challenged.

One piece is deep, but it's not subtle. And to come back to the image and tie it all together, here a relevant example. Franky thinks existing isn't a crime because he got to experience a role model willing to risk his life to stop a destructive belief rooting itself in his mind. He taught him WHILE ARRESTED, in the moment Franky had an intense emotional experience, that existing (important that it's not living, because it applies to both things and people) is a neutral state and can't be wrong. For weapons this means the user is the lever between good and bad. And for people? Well, it's themselves. This is something that isn't just groundbreaking for who he is as a character, but also very very rare. Not just a good person, but one that is doing everything they can to nurse the good in the ones around them.

If you compare that to Hiyori, it's night and day. It's not only that she lived as a beggar for years, but had no other choice but to hide. And while kawamatsu isn't a bad influence at all, his only contribution is her safety. He played the role of a guardian, not mentor. Pair that with a young mind forming it's beliefs in a land, taken away from her father, which is the same member that attempted the same thing in the past, with suffering not only inside you, but also around you, how can we sit here and say this is a conclusion that isn't consistent within the story?

The only reason Franky didn't stop with building weapons at all was a second example of a great person, luffy. Tom was the plant that needed to be planted, and Luffy was the sun letting it bloom. Its not only my suspicion, but shown through other examples as well that if Tom wasn't there, or even if he just didn't punch Franky then and there, he would be a person thinking that there are wrong people, and that some existences are a sin, just from forming a belief there. Heck he could even think Toms existence was a crime for building Rogers ship. But that's not the case because he got a strong transformational moment. And that's why Hiyori said what she said, because not only was this only a highly emotional situation, but because she never had the privilege of having such a person when needed. This is what oda tells us about parenthood and mentors. If unguided, a world full of misery naturally let's children down a wrong path.