Alma 14:3

And they were also angry with Alma and Amulek; and because they had testified so plainly against their wickedness, they sought to put them away privily.

They were angry with Alma and Amulek because they had spoken so clearly against their evil ways. They tried to secretly get rid of them.

The fact that Alma and Amulek had spoken so plainly about their wickedness made them even angrier. They wanted to kill them in secret.