@kazaddum
I say ass-backwards because he's spitting in the face of what they value.
1. He is freshly declared their leader by birthright and unproven.
2. He denies them their post-battle executions. If I'm a hungry warmonger, I expect to see my enemies ceremoniously executed. Instead, he provides them transportation, funding, and armed escort.
3. He forsakes Jomsborg. Jomsborg is a major fort of the Jomsvikings. He announces they will strip it and bury it to the ground. Weapons will be buried, slaves will be released and given funds. Sounds like Thorfinn hates fighting, slavery and is unfit to be their leader.
4. Thorkell claims that he has the authority to settle the Jomsvikings' dispute by order of Canute. He then says that he's relinquishing his authority to Thorfinn. Honestly, it sounds like a sleazy bluff to declare Thorfinn as legitimate.
5. Probably the hugest problem here, he disbands the Jomsvikings. He's denying them their very livelihood seconds after being declared their leader.
All of this adds up to reasons to resent Thorfinn. Although you say they value loyalty, I disagree. From how they have been depicted in the story thus far, they value battle. And Thorfinn has done everything possible to deny them that lifestyle.