Theophylact of Bulgaria
Woe to them, because they have gone in the way of Cain through fratricide. For by imparting impious teaching to their brethren, that is, to men of the same race, they kill them by their evil doctrines. Or again, by consuming the seed, they kill their brethren potentially, that is, those whom the seed, brought to maturity, would have brought into life. They have gone in the way of Balaam, because they also do their work for gain, as Balaam did. They have gone in the way of Korah, because, like him, though unworthy, they seized the dignity of teacher for themselves.