One of the secondary antagonists in Dragon Age: Inquisition, Raleigh Samson led the Red Templars during the mage/templar war, rallying them to rise against and overthrow the Chantry, which had gotten them all addicted to lyrium. In service to Corypheus, General Samson introduced the templars he rallied to the red lyrium Corypheus supplied to them, creating a different brand of addiction that would forever change -- and in many cases corrupt and destroy -- their lives.
Prior to the Inquisition and the events in Kirkwall that inevitably led to the mage uprising, Samson wasn't actually a bad guy. If anything, Samson was a good guy with a soft heart, and it was that very softness that led to the eventual destruction of his life.
Samson joined the templars prior to Knight-Commander Meredith's rise to power, but when she took over the order in Kirkwall, she discovered that Samson had been delivering messages for a young mage named Maddox to his lover in Kirkwall. Meredith removed Samson and left him to live to the streets, either not taking into account what the sudden removal from his lyrium supply would do to him, or simply not caring.
In Dragon Age 2, the player is directed to meet with Samson in the alleyway near the docks while searching for a missing mage named Feynriel. According to a templar named Thrask and Feynriel's father, Samson was a "'retired" templar who helped mages out of a sense of guilt. Samson claimed he directed the boy to a Captain Reiner, who occasionally took on refugees and might actually be holding Feynriel hostage.
Sending the boy to Captain Reiner may not have seemed like the kindest action on Samson's part, but by that point, he'd been suffering from his addiction and struggling for a means to find lyrium. He'd been panhandling in the streets for quite some time, and when Hawke next encountered him after returning from the Deep Roads with a boosted fortune, Samson actually asked the player to help him out.
When Hawke encountered Samson again in Act Three, he teamed up with Thrask to overthrow Knight-Commander Meredith, but was offered the opportunity to become a templar again by Knight-Captain Cullen if Hawke recommended he be given another chance. After the mage uprising and the fall of Knight-Commander Meredith, Samson rescued his old friend Maddox from the Circle, though by that point Maddox had already been made Tranquil. He also helped several mages escape the Circle before he rallied the lost and disillusioned templars to his cause. He urged them to rise with him to overthrow the Chantry, which had ruined a good many of their lives by addicting them to lyrium.
During this time, Samson began to serve Corypheus, and he introduced his fellow templars to red lyrium. Samson knew relatively quickly that this was more potent (and more addictive) than regular lyrium, and that it had dangerous side-effects that corrupted both the minds and the bodies of those who consumed it. Samson was one of the few who seemed to be all but immune to these side-effects, gaining the benefits and power red lyrium provided without losing control. As he continued to take it, he grew more powerful and vengeful.
Willingly serving Corypheus (if the Inquisitor recruited the mages prior to the attack on Haven), Samson led the Red Templars into battle against the Inquisition, claiming a victory that only bolstered his resolve. He formed a base at the Temple of Dumat, leaving Maddox there to continue researching the red lyrium. If the Inquisitor investigated Samson on Cullen's recommendation, they infiltrated Samson's hideout to learn how to defeat him in battle. Maddox committed suicide to protect Samson, but the Inquisition found the plans he used to create Samson's armor, returning it to Dagna so she could create a rune that would disable the power of his red lyrium armor.
In a final showdown with Samson at the Temple of Mythal, if the rune was acquired, it gave the Inquisitor a significant advantage in the fight. Otherwise, they faced him at full-strength. Upon defeating him, Samson was taken prisoner by the Inquisition, and the player character was tasked with determining his fate. The player had five options for how to handle Samson: hand him over to Kirkwall, conscript him into the Inquisition, allow Dagna to study him, exile him to the wilderness or life imprisonment.
Regardless of how the Inquisitor chose to deal with Samson, what remained of the life he tried to recapture after Meredith's downfall in Kirkwall was swiftly destroyed. He spent so many years feeling powerless as a slave to the lyrium addiction the Chantry forced upon him when he took his templar vows that the first opportunity to conquer that addiction and turn the tables in his favor corrupted the last bit of softness in his heart. He knew exactly what he was doing when he joined forces with Corypheus, and the fact that he would seemingly never go without lyrium again in the Darkspawn Magister's legion cemented his decision.
What happened to Samson after the Inquisition is impossible to say because, eventually, the Inquisition was forced to disband. Whether he remained imprisoned, worked with Cullen, wandered the wilds craving lyrium he could no longer obtain or imprisoned in Kirkwall, one almost hopes they'll see him again in future games to learn if he ever conquered his addiction and rediscovered the softness that once prompted him to befriend and help a mage remain connected to their life before the Circle.