DanieBarton03
Odin and Septum both allude to him being part of the organization, though it sounds more like he was just a freelance assassin more than an actual member. Odin also mentions, when talking to little Hee-chan, that he regrets killing Heero and left the organization right after. It's not directly mentioned, but the reader can assume that Odin was taking orders from the colonies' side to kill Septum.
Or so he thought: it was actually all a trap set by Dekim. A
three-way trap, in fact.
The Quinze-led attack on the command center of X-18999 is just a diversion, one orchestrated by Dekim so that Odin Lowe could sneak into the command center. General Septem obviously suspected that Dekim might be surreptitiously helping the rebels, too, as evidenced by Septem's sneer, "Hmph! Quite the weasel, aren't you... This is why you can't trust anyone from the colonies!" The attack was designed to fail from the get-go, honestly: the Specials were in Dekim's pocket from the beginning. He expected them to overpower Quinze. That's why, once Odin makes his move, he lies to Quinze and tells him that the communications lines have been re-established so that Quinze will withdraw and save what's left of his forces. A rebel defeat would also be a great feather in the Specials' hat -- thus elevating their status and bringing OZ's sleeper agents ever higher in the pecking order.
But why kill Septem?
When Odin calls out to Septem before he takes his shot, Septem knows exactly who Odin is. He's also surprised that Odin was still taking contracts after he'd commissioned him for a mysterious "job" that he does not expound upon. Then, when Dekim confronts Odin, he shoots him for "revenge" rather than to "silence" him. Odin realizes Dekim's plan at the end -- he comments on the "irony" of it.
And there we have it.
Septem was the one who hired Odin Lowe to kill Heero Yuy.
Dekim paid Odin to kill the General to put both players in the same place. If all went well, Odin would kill Septem, and then Dekim would kill Odin. Two birds, one stone. The leader of the colonies would be avenged... at least in part.
The fact that the attempt on Septem failed probably doesn't matter all that much to Dekim: he's a patient man. He'd also scored two major points, in that Odin Lowe was now dead and that the effectiveness of the Specials had been proven. Once "Operation Meteor" was underway, Septem would probably get his anyway.