What quality would 勝手 describe in these contexts?

Question

I looked it up in online dictionaries and they didn't really prove useful. "Selfish" or "convenience" doesn't seem to fit.

So this is a quote from Niji-iro Tohgarashi

勝手な奴だ。

This is the sentence a brother said about his sister, after he found out that much as she hated insects, she seemed to like fireflies (which are insects), a lot. I figure what he means is "she hates insects, yet she likes fireflies which are insects, so she's kinda self-serving in a way, likes what looks beautiful and hates what doesn't". But "selfish" sounds a bit misleading/confusing, and harsh, and I'm not sure about the connotation of "self-serving" either.

Yet, the same word is used later

害虫も雑草も人間の勝手な都合で分類されたもんだろ。

In this case, it describes the way people classify insects and grasses into categories like pests and weeds to their liking. They like what's good for them, and hate what isn't. In this context, both "selfish" and "self-serving" sound apt.

If both of these contexts were independent from each other, I would go with "Whatever suits her" with the former sentence, and "The selfish means" with the latter. But they are not independent within the bigger context of the manga. And both sentences use the same word. So I wonder is there a good way to describe those similar qualities ("a little self-serving, but not that serious" and "it's self-serving and now we're talking seriously") with the same word(s) in English that will sound natural and soothing for both sentences.

Answer 1

勝手 can also be taken to mean arbitrary. Instead of directly referencing the definition of 'selfish' with all its English connotations, it's easier to think of it as describing the basic idea of just doing as one pleases; without approval, without auxiliary, and with their own judgement.

While 'arbitrary' doesn't carry the negative connotation that both sentences have, it is unfortunately the only word I can think of that stays true to the meaning of the original and works in both contexts. Hope this gives you a place to start off in.