I think the layout of resources, NPC, crafting stations in all current cities will facilitate killing inside the cities. They need to take a serious look at this.
There are a couple things they should do to make it still possible but harder.
1. Calling guards when there is no criminals still calls a singular guard over, who then berates you over calling guards when there was no crime committed. Doing this multiple times just makes the same guard come over, he should stay in the area for a short while before returning to his old patrol. After X amount of times, calling guards stops working for Y amount of time. The entire reason for this is incase you are at the crafting tables, bank or doing literally anything and someone is clearly setting up to kill you and you are about to need the guards, having a guard present when you get killed saves you having to call guards while it happens as guards call other guards to them like in Mo1(I hope) so you can focus on defending yourself.
2. Items dropped by blue(or whatever the new reputation system will mark good standing players as) players being killed inside of cities/guard zones are marked as stolen items for anyone not the owner(or same guild). This to prevent the good ol' Sacrificial lamb looting the blue lootbag to then die, giving a non criminal the ability to loot the grey lootbag with no consequence. Same rules to stolen items as thievery stolen items apply, making it so you can not bank these items while they are still flagged and being in possession of one, guards will attack you if they find them on you.
This means if you want to kill a player in a guard zone you have to be sneaky as not to alert your target, be fast as well as have a escape plan. The same way thieves work but a bit more bloody with potentially bigger rewards.