Is this the right answer?

Because Jamis was going buck-wild from a Fremen perspective, which is why Stilgar told Jamis that after his fight with Paul, Stilgar was going to kill Jamis himself for his actions.

To piece the why of this, when they first found Paul and Jessica, a little confrontation occured where Jessica prana bindu'ed Stilgar into a hostage position (I believe knife at throat or behind him able to snap his neck) while Paul decked a Fremen, stole his gun, and ran/climbed to a high cover position to facilitate Jessica's escape if a bargain couldn't be struck.

A bargain was struck for both to join Stilgars tribe, but Jamis, the decked Fremen let the shame of Paul making him look weak gnaw in him over hours, and started invoking religious law for the specific purpose of salving his wounded pride by forcing a 1v1 fight with h Paul to the death.

To the Fremens, their beliefs and rituals were sacred, and this assholes invoking it because some kid for the better of him for a second in a dark cave? He's invoking the word or God and prophecy so nobody can stand in his way of murdering a kid half his size?

You can bet Chani and Stilgar were on Paul's side. Jamis was a full blown blasphemous petty bitch. Among other things.