Is it just me or does Offence as a bonus trait feel wrong for an Arbitrator?

Defence seems more in line with the fluff of the dogged arbitrator.

Try look at the description of Offence and Defence on page 51 and tell me which one screams arbitrator?

Depends if the character was part of a Riot Squad or just a detective. Being a part of a Riot Squad would most definitely warrant Offence instead of Defence.

Looking at the recommended roles for the Arbiter: Assassin, Desperado, Seeker and Warrior.

The riot squad will be "warrior" Arbites. The warrior role both has Offence and Defence, so changing the arbiter from offence to defence is not going to change anything.

The seeker which is the most archetypical detective has neither. And it doesn't make sense that the detective is a melee frenzy specialist. Defence seems much more in line here, to make them the hardened 40k investigators I envision that they should be.

If we intend to keep them slightly in line with the DH1 Arbites, Defence is the way to go.

Why not make the choice between Offense and Defense a this or that option?

Much like DH1e had either this or that Weapon Training Talent, DH2e could go a step further and incorporate a choice of either this or that Aptidude. Of course this means taking a good, long, hard look at what the Aptitudes affect, which means too much time investment...so, never mind.

If you like the suggestion, take it and House Rule it. IDC