Probably the issue that drove discussion the most during the first few years of the games existence, and the cause of the expansion that created the great schism known as Trammel.
The issue hasn't gone away. Countless free/player shards now face the same dilemma that confronted the UO dev team in the late 90's. Namely, how do you discourage Player Killing without destroying it and the unique nature that it's existence gives to Ultima Online?
Why not just destroy it? Because it is an essential element of what many of these shards are trying to recreate. The danger, the risk vs reward, the player justice that made old UO great.
So how do you find the balance? It's difficult. The most bold attempt so far has been the effort of the IPY2 staff, with the introduction of the player driven Detective and Paladin systems. You have to applaud the ideas(as well as the coding) that resulted in two very polished and interesting systems being introduced on the shard. Ultimately, they have failed due to the usual nature of UO players - the systems have been exploited to the point where they are much more of a problem than the thing they were designed to address!
The AOS rule set provides the easiest option I believe - insurance. On AOS:R we have introduced increased insurance costs for red/murderer characters. Currently set at 4x the normal rate. The theory is that this amount presents a serious barrier to people intent on ruthlessly murdering - the chances of it being a profitable profession are greatly diminished, meaning many players will simply choose to forgo that play style. Those that do continue on this path will presumably be the most skilled and dedicated of players.
Obviously, it remain to see in play how this will work. We may need to adjust the cost ratio. We may also need to introduce other methods if this becomes insufficient deterrent alone.