Alright, so here's how I look at this:
Looking over the rules, let's understand that nothing in the MOTD says anything about not being able to defend yourself if you're being restrained (unless under fear rp). In this situation that occurred, it would have been FailRP for the HSU to arrest you since you were under the disguise of a CPL (still getting an answer from the HSU on the reason) but, for attempting to do so, you defended yourself. In this context, I take the rule on D-class not being able to kill each other (which many still do) and apply the integrated self-defense rule in this. D-Class personnel can protect themselves if they're being attacked (agreed among SMT). As staff, we should only be going after the one who instigated the fight (I'll probably note this down in the motd at some point). I've had plenty of situations myself where someone was attempting to cuff me over no justifiable reason and I've killed them for it because there are no rules against it.
But let's switch our position. Say some low ranking personnel ended up killing me instead... usually that would be considered RDM for attacking another foundation personnel and it would be 'justified'. But in the rules, we know foundation personnel are not allowed to harm others... This has happened many times before over my years in staffing and even now. I see that this is a problem within the rules that we, as SMT, have to fix. As Mind said, the situation was poorly handled and each party involved could have handled the situation better. So, was this RDM? Yes AND no. This is no longer a case where it's a simple "It is" or "Is isn't".
Due to the confusion, I will lift up the warn. As I've stated clearly many times, there are no rules against it. But there will need to be a change in the rules to get this cleared up more.