It's an explanation for why what she did wasn't unjustifiable.
How? You left out anything approaching a justification for her actions.
Justification
noun
noun: justification
1.
the action of showing something to be right or reasonable.
Centuries of other people doing bad things to other people doesn't make the slaughter of different people uninvolved in those actions any more right or reasonable. The weakness in that argument is the false attribution of responsibility, not the time period: a racistequivalence fallacy that holds all humans responsible for the actions of some isn't any more right or reasonable- even before we address the point that her assumption is kind of stupid.
Vellana's response to believing that Group A (humans involved in the attack) was responsible for killing favored Group B (her group) was to kill as much of Group C (all humans) as she could as they came across her path, because Racism somehow means that Group C and Group A are equivalent. That is pretty much the height of unjustified revenge.
(And, for the record, what Zathrian did was also unjustified and stupid as well. You're right that they weren't so different- they were also both wrong and unreasonable.)