Reason; can't move forward without this

From the moment Lockon Stratos found out who was behind the terrorist bombing that took the lives of his parents and sister, he must have thought about revenge. Even though he spoke to Setsuna, upon being aware of KPSA and Ali Al-Saachez, like he was willing to move on toward the future, the reality was that Lockon was still not ready to let go. He could not let the person who ruined his family and childhood get away without retribution. And this would mark him for a short and bitter life.

For Lockon, it comes down to being able to move on. He feels in order to move on, he has to take revenge for his family. He has to make Ali Al-Saachez pay. Before he sets out to his demise, he says to himself, to move forward, he has to do this, otherwise he can't face the world. He is fully aware, whether he does anything or not, nothing will change. But it is the part of him that refuses to let go of the past, the part that dwells on what has already been lost, that leads him to his folly. For Lockon, in order to see the future, he had to confront his past.

So when he sees Ali Al-Saachez in battle, Lockon rushes in to fight him, even though his dominant eye was injured, and he thereby had no way of making his usual clean snipes. When his Gundam is struck down and heavily damaged, due largely to his blind spot on his right, instead of retreating, Lockon charges full force to take down the man he hated most. Even when Ali Al-Saachez completely destroys Lockon's Gundam, Lockon refused to go back, leaving his Gundam, and the precious GN-Drive to Haro. As Haro is forced to return Dynames to Ptolemy, Lockon makes a fake promise, saying "I'll come back alive." I say it is a fake promise because I don't think Lockon believed he could return alive, but that didn't matter. The moment he stepped out of his Gundam, armed only with his sniping module, he made his choice to die trying to kill Ali Al-Saachez, his family's murderer.

When he has done all he can in his attempt to kill Ali Al-Saachez, Lockon drifts away in space, bitter at the world that turned a blind eye to terrorism and hate. In one of the saddest moments in the series, he sees the Earth below him, and wonders if the people there are satisfied with the world they created and left in disarray.