He had a horrible kit-hood and gave his enemies what they deserve. His siblings bullied him as kits, then asked him for help later in life. He killed them, like they deserved. You don't bully someone then plea them for help. Of course they won't help you, act like a jerk to someone and they'll eventually act like a jerk to you.
Tigerstar is the same case. He beat Scourge up when he was young, then asked him for help to take over all the Clans! Yeah, allow him to help the cat who basically nearly killed him as a kit. Uh, no? So, he killed him, too. Like Tigerstar deserved. (Then he tried to take over the forest himself.)
Sure, they could have forgotten about the time they bullied Scourge, but I'm pretty sure that's just adding salt to the wound with Socks and Ruby. You don't just forget about the brother you bullied and made fun of as kits. Tigerstar is more understandable, but he really should have gotten some ink of recognition. I mean, Scourge recognized him.

Scourge's life was basically karma. Tigerstar, Socks, and Ruby all treated Scourge like trash. As karma, they were killed by Scourge.