You have picked up a good amount of the game mechanics thus far, but a couple things that are misleading about the game have gotten you a bit confused

1 - Armor. Your level of protection vs slash/bash/pierce grants you a higher chance to be *missed* by low level mobs, and players with VERY low hitroll. This is not what makes certain classes good at protecting themselves. Almost always, these protections come in two ways;First, the form of damage reduction (healers/paladins/shaman get sanctuary to decrease incoming damage, most mages get varying forms of damage reduction as well). Secondly, characters defend themself using "defense skills", such as dodge, shield block and parry. Your human assassin has all three of these skills, and with all three firing off at once, you will definitely defend yourself well (once these skills have improved somewhat) .

Sidenote: You will have a use for the saves vs mental, spell, paralysis and petrification; these do not fall under my above note.

2 - Evil PK. The idea behind evils is that they want to improve themselves. Therefore they are more likely to strike out at other players without much warning, and since a lot of the better PK'ers don't always want to be restricted in who they can fight...yes, on average more of the higher skilled players will play evil. However, evils DO attack evils, if it is beneficial to them. They just see goodies as weaker often, so are more likely to attack a definite enemy that is weak, than a possible ally that may or may not be weak.

3 - Merchants : Merchants start with a limited amount of money, so the ones that do not get a lot of business will be poor often. If you can discover for yourself which ones actually get good business (hint: You mentioned always buying a return potion), find things that these merchants will purchase to make some coin. Most merchants will buy things that are similar to what they sell.