in on an manslaughter beef

Hi.

I got this puzzling sentence. I'd appreciate your help.
This line's from the movie, Con Air, starring Nicolas Cage.

Cameron Poe(Nicolas Cage) accidently kills a man while trying to protect his girlfriend and now he is being transferred by plane to another facility with other convicts. As these convicts killed guards and took the control of the plane, authorities try to come up with a solution. They're having a discussion. A n agent starts saying Cameron Poe used to be a US marshal, highly decorated, defending him and his boss asks him back why it matters. And he answers,

We got a plane up there filled with killers, rapists and thieves and
we got this guy Cameron Poe, in on an involuntary manslaughter beef.

Could you help me understand what 'in on an involuntary manslaughter beef'?

I can see he keeps talking in favor of Poe but can't tell what exactly that means.

Thank you in advance.