Since as a guardian you're a devout defender of humanity your level could be tied to your "humanity" which you lose a little of each time you kill a fellow guardian. Kill 2 or 3 and you lose so much humanity that you regress a level (you do not get to reallocate your skill points, they're locked, but if your new level is lower than what is needed then you lose the ability to use that skill until you level back up. This is also true for any items whose requirements you no longer meet). This would allow friendly fire, discourage trolling, and give a realistic/lore based consequence for intentional/unintentional for killing a fellow guardian outside of PvP.