Sam doesn't consider himself a particularly violent guy. He's generally easy-going and tries to get along with everyone as long as they don't throw slushies in his face or hurt his family. And there's the rub, his family's been hurt.
He's fought for Kurt before and will always do it because Kurt was the first to offer him friendship and the only one to sacrifice his own happiness to protect him.
He was happy to defend Kurt from Karofsky - the image of that hulking mass going after Kurt makes him ill - even though it was pointless in the end and he wishes Kurt had told them earlier because no one should have to go through shit like that alone.
So yeah, he took on Karofsky - unprompted by the way - so there's no way in hell he'll let Blaine get away with hurting his family. He knows he'll be disrespecting Kurt's wishes and the disappointment he'll have to face is going to just suck, but at least he'll go to bed tonight knowing he did right by his family.

Blaine is emptying his locker when Sam catches sight of him and he stalks up to the boy, slamming his fist against the locker.
Blaine twirls around in surprise and his glance flitters from Sam's fist to his face and back to the fist and the dent in the metal.
"Kurt doesn't want you to hurt me.", he stutters rapidly, color rising in his cheeks; "so, you can't."
Sam may not have Finn's height but he can still loom, so he does. Sweat is starting to bead on Blaine's brow as he tries to lean back without looking like he's actually intimidated.
"You hurt my family. I don't take kindly to that."
He almost scoffs when he sees the confusion in Blaine's eyes, leave it up to him to not realize that
Kurt is as good as his brother now after everything he's done for Sam including opening the Hudmel
house to him - because Carole may be the Mom now, but Kurt is still the head of the household.

Sam glances around at the empty hallway, grabs Blaine by the strap of his bag and drags him in the bathroom, throwing him against the sinks in the far back. Blaine stumbles, catching himself against the wall and quickly rights himself, clenching his fists. His mouth folds in an arrogant smirk as he hunches a little, taking on a boxer stance.
Sam knows about the boxing and he knows about the fight club but Blaine doesn't know how Puck pounded him on the back after the Karofsky thing, mocked his fighting skills and dragged him to back-alley brawls to teach him how to really survive a rumble.
He's ready.

They eye one another and tension mounts. Sam is about to feint to the left to deliver a kick to Blaine's knee when the bathroom door slams open.
"Sam!"
He doesn't turn around, because if he looks at Kurt, all his resolve will crumble.
"Go away, Kurt. This is between him and me."
Out of the corner of his eye, he sees Kurt draw level with him.
"No, Sam."
It's like there's a glacier at his side, everything about Kurt is cold and white and so different and Sam could just cry because Kurt won't.
"He hurt you."
Kurt shrugs. "Many people have. You can't beat them all up."
Sam's aghast but Kurt remains impassive under his horrified stare.
"Not like that, Sam.", he says, his eyes softening a little. He hasn't looked at Blaine at all, ignoring the boy completely. "I'm going home. Finn has practice today, so unless you want to study in the library you can drive home with me."
Kurt looks steadily at him and he hesitates, rage still burns inside of him, adrenaline still surging from the burgeoning fight. Kurt is cold, wide open spaces, fields of ice all jagged and deadly and the fire in him dies.
"Okay."
He turns to grab his vest where he dropped it when he hears Blaine scoff. The banked fire flames up again and he whirls around and kicks the boy square in the nuts. Blaine drops to the ground and vomits.
"Sam."
Yeah, there's the disappointment. He'll deal with it. He lays his arm across Kurt's shoulders and guides him out of the bathroom, leaving Blaine to writhe on the floor.
"Let's go home."

tbc