This is the opposite of my read on the Splinter Cell series. Fisher has no tendency to torture in either 1 or 3. What he does is grab a dude, hold a gun or a knife to him, and say he might kill the guy if he doesn't cooperate (give Fisher information). In 1, you also don't have the option of pulling the trigger on the guy, you can only knock him out. Maybe I just have a different definition of torture to you but I wouldn't consider pointing a gun at a guy and asking for a door code to be torture. In Conviction, where you hold a guy's face in a fire for 5 seconds, that's torture.
In Chaos Theory (which is pretty much the game where Sam starts having a personality) he's shown to be disgusted by torture and torturers. The first mission has a scene where you witness a guy hanging over a bath being tortured to death with some electrical device or other. If you "interrogate" (this is the hold-knife-to-throat-and-ask-questions routine) the torturer, Fisher angrily wants to know why he tortured the victim, and when the guy doesn't even know, says something like "You're just the stupid guy who likes to hurt people? Can you think of any reason the world wouldn't be a better place without you?" (whether you kill him or not is, as always, optional).
Also, though Lambert orders Fisher to leave the torture victim's body where it is (hanging, bleeding into a bath), the player has the option to cut the rope and lie the guy down in the bath, saying "Just because he's dead doesn't mean I need to leave him here hanging like a piece of meat". Lambert tells him to stay rational, and Fisher says "You can spare 30 seconds for some simple dignity".
Stuff like this is why it seemed like such a bizarre turnaround in Conviction and to a lesser extent Blacklist when Sam just tortures and kills everybody he meets (I'm pretty sure there's only maybe 3 guys you need to kill in the entirety of Chaos Theory).
Semi-related: one of my favourite things in Chaos Theory is the scripted sequences it has which can play out differently, including different voice acting, if you try to go off-script, and it's never signposted. Like the time you're ordered to call in an airstrike on some crashed US planes whose pilots are still alive in the wreckage (I guess in order to destroy some data they were carrying before the enemy can get it). There's no hud pop-up to tell you it's a Moral Choice Moment or anything, but you can sprint off down the street and get the pilots out of danger before you call it in if you're quick, with the short term repercussion that your boss yells at you about it.