In a very literal sense, Willy is solely responsible for his own destruction. After all, he did kill himself (or at the very least, that is what can be assumed when he drove off in the car and later found dead). He had a history of suicidal attempts, crashing his car on numerous occasions. An insurance inspector had to inform Willy's wife Linda that they had evidence that all the accidents Willy Loman got into within the last year of his life were in fact not accidents at all. There was even an eye witness, a woman who saw Mr. Loman purposely smash into a bridge railing. From this it can be inferred that Willy's death was indeed a suicide in the end.
In a more figurative sense, American society can be blamed for Willy Loman's demise. The media plays up the business world, claiming great amounts of money are to be had. Willy Loman was just another man sucked into this dream and, in a way, the business world drained his life from him. It is said he became a salesman when he was a boy, around eighteen or nineteen, and stayed in the business up to the point of his death, which occurred the day he was finally fired from the company where he was working. Willy was around the age of sixty-three when that happened. That means that he put the majority of his life into the business world, searching for riches and ever elusive respect. Approximately 45 years of his life he spent searching for these things, and he never found them. Tragic? Yes. But a reality Willy Loman had to face. Perhaps it is these things that led him to try committing suicide numerous times. He was a very unhappy and unsatisfied man, who'd lost most of his hope. He'd invested a lot of this hope into his son Biff too, who unfortunately let him down as well. Willy had such high hopes for Biff becoming the success he himself could never achieve, yet Willy failed to take into account what his son desired. This is not unheard of when it comes to parenting. In fact, it is quite common.
In essence, the American Dream promised Willy Loman things that could never be obtained. He spent his whole life endeavoring to reach them, and it was a factor that led to his demise. As to the question of who is personally to blame for Willy's destruction, it can be seen that Charley, Biff and Howard are the vehicles through which society acted to tear down Willy's hopes and dreams, and, eventually, his character. More abstractedly, the makers of American society, the people who established the ideals of the time, and the ones who ran the country were responsible for this small man's death. Willy Loman never quite fit into the mold set out by society, even though he tried his damnedest, and in the end, his efforts destroyed him.