But if you accompany his saga to earn his knighthood back in DA2 and become a templar once more, you actually learn this is a mistaken notion people have formed of him.
During the entire game, he complains the citizens of Kirkwall look down on him. He says that, if he were in the order, he'd be respected. He complains his addiction to lyrium, forced by the Chantry, stole his will to fight.
I only got the thirst and the dust.
And his complaints are not just the rambling of a weak man. He was indeed expelled for the Order for doing nothing wrong. He only sent love letters from Maddox, a mage, to his girfriend. While templars like Ser Alrik get to abuse mages repeatedly and suffer no punishment, Samson gets expelled for this. The punishment was disproportionate to his crime.
This is why he helps Hawke uncover the rogue mages in Best Served Cold. He wants to have a job. He wants to have a stable income. He wants work and his dignity back. This shows he's not a degenerate, a junkie, as he was portrayed in Inquisition. He never wanted to become addicted in the first place. For an act of kindness, even if a mistaken one somehow, he got severly punished.
One can only imagine what sordid things he had to do not to starve to death. How many nights he spent alone, enduring the withdrawal pains as he struggled to survive. The reality of a drug addict is quite familiar to him. We see this later on in Inquisition, as he sympathizes with the templars who were abandoned bt the Chantry and had to struggle with their addiction as well.
Every one of those templars would've suffered until nothing was left. And then, be forced to kill and die.
I gave them hope, just like the Chantry. Just like YOU.
Samson in Inquisition
The problem is that most people only see him as the degenerate junkie (he appears to be) in Inquisition. Cullen can't see past this, the player can't see past this. All they can say is that "he was weak". The thing is, this weakness was the byproduct of an unjust punishment, coupled with a lethal addiction and ten years living in subhuman conditions.
Also, one must bear in mind Samson probably had no luck finding a decent job -like Hawke did - simply because he had an addiction. Who would hire a drug addict? Nobody, let's face it. Which is why he turned to smuggling mages as a form of living. His situation was that desperate. Lyrium utterly destroyed his life and he knows it.
Being force fed lyrium by the Chantry was good for something
Is someone weak after surviving all this? I believe it takes a lot of strength not to slit your wrists and live to see another day.
He went through ten years of a neverending nightmare. It only ended when Corypheus offered him a prestigious job and means to help him with his addiction. The way I see it, he was in no condition to say No.
Ser Barris and the other templars were, because they were in the Order, with safe jobs, a steady income coming into their pockets and all the lyrium they needed. Samson? He had only 2 alternatives: keep crawling in the gutter or finally have a normal life.
And Cullen says he could've fought. Fought what? He's been fighting more than ten years only to stay alive. When you fight to survive, everything else is moved to second place in your list of priorities.
Samson needed 3 things: food, a job and lyrium. The Templar Order refused to take him back. Corypheus had all 3 of them. What was he going to do? Say No, because you're an evil magister? He only learned of Corypheus' intentions after he joined him. And if he fought him then, he would likely be killed.
If Cullen, Ser Barris or any other virtuous character had endured ten years of suffering as he did, only to be abandoned to their own fate, their own devices, their speech and actions would be quite different. You can only fight if there's something backing you up. Otherwise, you're a slave to your needs. And other people judge you for it. They see you as weak, even if you can't control the circumstances of your life.
Samson did fight for ten years to have a decent job and a normal life, as shown in DA2. He fought and lost, in his own words. Weakness is a very relative thing. It's in the eye of the beholder. To Cullen and many other templars who NEVER had to fight starvation, poverty, unemployment and forced withdrawal, Samson is weak. To the templars who were abandoned by the Chantry and earned their shield back, even at the cost of ingesting red lyrium, he was their saviour. A matter of perspective. A perspective that Cullen and other such virtuous characters lack.
But I'm weak, and you're a saviour. Do what you want. I'm done talking.
In the end, it all comes down to this: when the system abandons people, they'll open hand of their high and mighty principles and convictions to survive.
People can't plan ahead without money, without a home to live, without a family to help them. They're slaves to necessity. There's no room for political concerns or moral considerations if your belly is empty and you don't know if you'll have enough food tomorrow.
To summarize
Samson is a good man who was expelled from the system and became a slave to necessity. He tried to go back to it and failed. His rage against the Chantry portrays his rage against the system.
Unsurprisingly, he was right. The Chantry crosses its arms while the Inquisitor has to risk his life to close the Breach. A former Seeker has to declare the birth of an Inquisition to find a way to save Thedas. And once the Breach is closed, the only thing they care about is who will sit in the Suburst Throne. The victims of Corypheus' army be damned.
It seems Samson always had his heart in the right place. Had Corypheus never found him, he might've been one of the first to submit himself to recruitment by the Inquisition. Anything to help an institution that just might be better than the Chantry and show it how it's done.