When I watch Jidaigeki (chanbara) I remember the saying 'Nusutto nimo ichibu no tamashii'.
It might not be an appropriate translation of the Japanese expression, but what that means is that even thieves have their own kind of ethics.
In one of the episode of 鬼平犯科帳, titled 'Ipponmayu'', the respected leader of the gang of thieves (whose name is the same as the title) said:
Thieves who steal by killing people are not real thieves - they are just murderers.
I think what he really meant was that the real thieves depend on their own impeccable techniques, which is only achievable by years of training and practicing (techniques are often passed on by the leader). Therefore they don't have to hurt civillians.
The character 'Ipponmayu' absolutely loaths those gangs who do 'Isogi-bataraki' ('quick jobs' where they break into 'Oodana' (big retail stores) and steal money by killing the owner family and every single employees)',
Even Yakuza gangs have the similar ethics that is not to hurt or scam innocent people.
Ipponmayu mourned over the dying ethics of 'real' thieves - and emergence of new generation gangs with little ethical values if not at all.
You might wonder how could a leader of a gang be respected - but he obviously was.
He was proud of his techniques (almost like an art of stealing if there is such a thing), thus not having to hurt innocent people.
A gang (of thieves) in the past was like a family (although, not always genetically related) and the leader was kind of 'father figure' to the members.
The gang shared special sort of bond and always worked as a team.
If you understand Japanese and a big fan of 鬼平犯科帳, you can see how Onihei could relate to Ipponmayu's view.