That's still way too much of a reach for me to say Walt is responsible for Combo's death.
Walt is responsible for Hank's Death. Walt is responsible for Andrea's Death. Walt is responsible for Gayle's Death.
Those and the people he outright put hits on and murdered with his own hands are on him. I'm just not seeing the Combo thing.
Walter wanted to expand to make more money, because of his growing reputation after Jesse Killed Spooge, he suddenly tought that he was one of the big boys, he needed dealers to do it as he's not a dealer himself, he's the chemist, the only dealers that he knew/could trust/get were Jesse and his friends who were dealers. Walter organised the team meeting with Jesse, Skinny P and badger about expanding and where they were going to expand to, Jesse didn't want him and his friends to expand out of the area, due to rival dealers, Walter was ignorant of this and thought nothing of it, the end result: Combo trying to deal on other turf and gets capped by boys who work for Fring, who additionally used a child to murder him, before killing that same kid themselves. This led to Jesse's downward spiral with drugs and Jane situation(combo's death that is), Walt didn't give a fukk that Combo(one of Jesse's best friends) had been killed, he was just another person in his eyes, just like how he gave no fukks about Todd killing the young kid. Money comes first for him.