What provision of the Compromise of 1850 particularly angered Northerners?

Answer: It was the Fugitive Slave Law.

Explanation: It was the Fugitive Slave Law. Indeed, it forced the federal and judicial authorities of Free states to aid Southern officials or slave owners to capture fugitive runaway slaves or be fined of $1000 dollars if they refused. It also forced citizens of Free states to participate in a posse to capture the fleeing slave if ordered to do so by federal marshals. Even Free blacks could be arrested as runaway slaves if a southern slave owner claimed property over them.