Both apply. Yes, he ultimately chose to do it. However society shaped and funneled him into that position. It's not a binary decision between individual and systematic, both can apply. In this case, social systems failed and put a large number of people in a bad situation with an apparent easy way out. Almost all then chose not to go on a killing spree. Unfortunately, "almost all" is "all". Some will make the bad choice, when put in that position.

As a society, we can't change individual choices. What we can do however is change the framework those choices are made in. If we aim to put fewer people in that position, then fewer will make the wrong choice, and we will all be safer for it.