why was the killing of the animals justified?

Basically they were killed for treason against Napoleon.  The pigs confess that they collaborated with Snowball in destroying the windmill and were planning to help Mr. Frederick overtake Animal Farm. They also confess to knowing of Snowball’s partnership with Mr. Jones for years. Then the dogs tear out the four pigs’ throats. Napoleon asks whether any other animal wishes to confess. Three hens, which had led the hen rebellion, confess that Snowball incited them to revolt in a dream vision. After this, several other animals confess to crimes both great (murder) and small (stealing). Napoleon has them all murdered.