I'm going to throw in the Black Dragon Society as an option. We know from them attempting to kill Victor that they're willing to send assassins to kill Davions that are (theoretically) working on their behalf. And the bonus is that if they kill off Morgan, they could possibly ruin Serpent, which would hurt everyone else more than Kurita, since the majority of Kuritan troops were with Bulldog. Serpent only had a single Kuritan Warship and a couple of DEST units. And if Serpent does fail, then the BDS could hope that Kuritan units were sent to finish off the Jaguars in the right proper, traditional Kuritan manner, rather than relying on barbarians.
Who's to say that the Black Dragons didn't hire an assassin themselves?
And lets face it, the idea that its WoB is pretty tenuous. IIRC, part of the explanation that its the WoB is the fact that "Only WoB could infiltrate one of ComStar's WarShips," which is pretty freaking amusing considering at the same time that they're saying that, there are a team of Nekekami who have infiltrated a ComStar WarShip to the exact same degree.
"Only the WoB could do this. And uh. Kurita. With their assassins over there, but only the WoB could do this! Don't mind the other 4 people who have infiltrated the WarShip, only the WoB could do this!"
And that could also explain why Morgan's assassin was killed by the nekekami. The Nekekami didn't recognize the assassin as someone that worked for Loki, he recognized him as a fellow Nekekami, presumably acting against Lord Kurita's orders, and was killed in order to ensure that he wasn't taken alive and interrogated the point where he compromised the other Nekekami team's mission. "Hi, yeah, I know you interrogated that assassin, and he revealed that he's a member of the Nekekami, but uh, just to let you know, I'm also Nekekami, but uh, I was sent by Theodore, and I'm not with the other guy, really..." Such a possibility would also explain why the Nekekami risked discovery and attacked the assassin. If you're a deep cover operative tasked by Kurita to run assassination/sabotage missions against the Jaguars, are you really going to risk discovery by throwing a throwing star? Its not like he has a personal attachment to Ariana Winston, she's just the latest commander. Redburn will do just as well, so I'd think they were unlikely to interfere and risk detection...unless the assassin had a Kuritan connection, and had to die in order to ensure that the Nekekami weren't compromised.
It would also explain why the 4 members of the Nekekami team had the same issues with their background as the assassin, because they used similar resources to create the backgrounds.