Id say terrorism is a tactic rather than an ideology. Using fear to intimidate the other side and targeting non-combatants. People have done that both to advance a political agenda and a human rights agenda.
Ah, but we weren't discussing what terrorism is, we were discussing what a terrorist is.
For example, a scientific researcher is someone who engages in research, but does engaging in research, make you a scientific researcher? No. Pretty much every country that's ever fought in a war or engaged in a military action, has engaged in 'terrorism' (Hiroshima fits your description of terrorism nicely, no?). But that doesn't necessarily mean they are terrorists.