RIKICHI: And you think they'll listen? Have you forgotten what we had to go through to keep the little rice that we have now?
He's stressing how little the bandits care for them: how few moral rules they have. Samurai the answer to their problem because they are principled enough to help a powerless village: exactly the kind of principles that simple mercenaries would reject.
GISAKU: What's the use of worrying about your beard when your head's about to be taken?
Gisaku's stressing that one's ethics or principles are only as good as the world lets them be. Sometimes, you have to make choices, and that might mean sacrificing a value you considered pretty important.
KIKUCHIYO: Looking at a worm like her I get sick. Wretched, helpless. I never want to be like that. I want to be reckless, daring...
KAMBEI: Then you just keep feeling like that until the bandits come.
Kambei echoes Gisaku's sentiments about ethics being only as applicable as the situations. If you have to give them up when trouble hits, then they're not worth very much. Kikuchiyo might talk a big game, but as far as Kambei's concerned, nothing counts until things get nasty.
KIKUCHIYO: Now who would have thought that this village held so many pretty girls!
A lapse of morals here from Kikuchiyo. Not that he's going to rape anyone or anything, but he's embodying the very fears that the peasants are so worried about. It's poor form and something a real samurai would never do; another thing he needs to stop if he wants to truly become a samurai.
KAMBEI: He gave himself up. He's confessed. He is begging for mercy. We must not kill him!
As a samurai, Kambei understands that you have to treat prisoners fairly. The peasants disagree. The movie certainly puts Kambei on a much higher level because of that, but it also wants to show us how useless lofty ideals can be sometimes. The peasants are gonna kill the prisoner, and nothing Kambei says can change that.
RIKICHI: You manage to kill all the samurai you catch, all right, but you can't kill the bandits?!
For Rikichi, morality is a matter of consistency. If the peasants are fine with killing individual samurai, why aren't they fine fighting a bunch of bandits? The rightness or wrongness of either half of the equation doesn't occur to him. He just wants them to apply it in all situations.
KIKUCHIYO: Look, you idiots. We come all this way and then look at the welcome you give us! Yet when I knock on your alarm a few times […] you all rush out screaming for us to help you! Stupid!
A rather simple expression of ethics here: treat your guests nicely. It tells us that Kikuchiyo is better than the peasant he was born as, as well as highlighting the ethical differences between the samurai and the farmers.
KIKUCHIYO: This baby. It's me! The same thing happened to me!
Kikuchiyo's realization here is the first real expression of sympathy he's shown. He's connecting to another human being in a profound way here, and the ethical package that comes with that (i.e., "you should care about other people") helps him make the spiritual transition from peasant to samurai.
KATSUSHIRO: I've always wanted to tell you how great I think you are.
A little bit of hero worship is okay, if said hero is the rock of samurai morality that Kyuzo is. For all the undermining of traditional Japanese ideas in this film, Kurosawa still wanted to emphasize that some things, like moral behavior, are still important.