In the Journal of Politcal Philosophy, Samuel Scheffler argues that terrorism is morally distinct from other kinds of violence directed against civillians and noncombatants.
[I]t does seem that terrorism is morally distinctive, at least insofar as it conforms to the pattern of what I have been calling “the standard cases.” In these cases, at least, it differs from other kinds of violence directed against civilians and noncombatants. By this I do not mean that it is worse, but rather that it has a different moral anatomy. By analogy: humiliation is morally distinctive, and so too are torture, slavery, political oppression and genocide. One can investigate the moral anatomy of any of these evils without taking a position on where it stands in an overall ranking of evils. Many people are pluralists about the good. We can be pluralists about the bad as well.
In the “standard cases,” some people are killed or injured (the primary victims), in order to create fear in a larger number of people (the secondary victims), with the aim of destabilizing or degrading the existing social order for everyone. The initial act of violence sets off a kind of moral cascade: death or injury to some, anxiety and fear for many more, the degradation or destabilization of the social order for all. Nor is this simply a cascade of harms. It is, instead, a chain of intentional abuse, for those who employ terrorist tactics do not merely produce these harms, they intentionally aim to produce them. The primary victims are used—their deaths and injuries are used—to terrify others, and those others are used—their fear and terror are used—to degrade and destabilize the social order.