There is no possible justification for the slaughter of innocent people, but what is the main root cause for these attacks that are spreading all over the world?
Ignoring the small "nut job" groups that have appeared over the years (such as the Red Brigade), the majority of incidents seem to originally derive from the treatment received by the Palestinians at the hands of Israel, and by association the western powers.
This is probably an over simplification of the situation, but to me it looks like a ripple effect that was started in 1947, and has been growing ever since, mainly due to subsequent actions by the west.