@Sep10trion - Typically elves devote themselves to a cause and have a good support structure. They are usually depicted as having low emotions, or a tight reign over them in order to maintain control. Much like a human, their memories fade over time.
Leo on the other hand has no support structure anymore. None of his fellow companions are alive, so he can't share his feelings to them. He can't really share his burden with humans, as they have such a short lifespan, that it would cause him problems if he emotionally attached himself to them (a typical trope that elves have). Unlike elves and humans, Leo remembers everything. So all of the time he feels useless over those 3000 years, it just continues to build up and weigh upon his mind, until it finally snapped.
Even then, he has the problem that he grows and evolves over time. I am sure if he continued travelling and living with his companions, he would eventually get tired of them. They would always have the same answers, the same mannerisms, and the same problems that they always had, as they do not have the potential for growth. It'd be like how you're in high school thinking about relationships and the complications about becoming an adult, and your friends are all talking about how awesome it is to play in that sandbox at the park every day. It is just not fulfilling anymore.
I wouldn't be surprised if it ends up that Leo killed a few of them.