Think about it, I think it's spelled out pretty plainly for us with Walter standing there with the pink sweater on. We're led through the chain reaction of cause and effects, poor decisions and their outcome. Walter's drive to achieve in an industry that he's completely unfit for (as was pointed out to him by a psycho drug distributor at one point) contributed to Jesse hitting the pipe hard, which led to his girlfriend falling off the wagon and getting on heroin again.
He was even there with the opportunity to save her life as she choked on her vomit, but relented. Her death then caused her father to drop the ball in his job as an air traffic controller, causing the collision of the planes, which brought Pinky the Bear to his pool, burnt and in pieces. This at the moment that he's contemplating how he destroyed his own family by the same decision to make bank as a meth producer and dealer; it's a metaphorical collision of disastrous proportions.
Don't forget the unseen victims, the users of his product, which has now spread across the whole South West, as demonstrated in this episode, it's comparable to a couple of airliners full of people colliding in midair, probably worse.