Killing Carl provides the best reward for that quest, an amulet that gives +4 to Vitality and Spirit, and +1 to the Evade skill. No level requirement.

Keep in mind some locked doors and chests can be picked with a high enough Lockpick skill. If your lockpick level is high enough to pick the door/chest, you won't use the key, even if you have it. Then it'll be in your inventory forever.