I didn’t want to emphasize any sort of anger this time around, but rather a sense of sadness regarding the decision the MIB made against his sibling. There’s a part of me that believes that while killing Jacob would have left him feeling liberated and free, he’d still be upset that he had to kill his own brother to do it. Considering the fact he knifed “mom”, Jacob was the last relative he had. And he wanted Jacob to leave the island with him when he begged Jakey-boy to go with him when the villagers finally made their exodus.
I got the impression the two still had some measure of a brotherly bond, hence they could still be civil to each other when they weren’t trying to one up each other. Just after a certain point in time, the resentment that the MIB harbored canceled out any feelings he once had for his bro. So consider this early in the time line.