Or HaChaim on Genesis 49:7/אור החיים על בראשית מ״ט:ז׳

1 ארור אפם כי עז, "cursed be their anger for it is very strong, etc." Jacob only cursed their excess anger. This is why he added: כי עז. Every person needs to harbour a certain amount of hatred otherwise he could not receive a reward in the Hereafter for having controlled his anger and made it subservient to his duty to serve the Lord. Jacob realised that he had to find a way to "cool" this anger which was liable to flare up from time to time. Inasmuch as both brothers suffered from an excess amount of fire in their genetic mix, Jacob hoped that by separating them from one another he could forestall future outbursts or at least make such outbursts of anger less harmful. This is why he added: "I will separate them within Yaakov, etc."