Full Idea
The most serious reason why a war might be unjustified is that it lacks any justifying aim at all.
Gist of Idea
The worst unjustified wars have no aim at all
Source
Jeff McMahan (Killing in War [2009], 1.1)
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.6
A Reaction
It seems that Louis XIV invaded the Netherlands in around 1674 purely to enhance his own glory. That strikes me as worse. I supposed Ghenghis Khan invaded places simply because he enjoyed fighting.