The Bible tells us to "put away" our anger and it associates what we might think as no big deal with other really ugly sins, like bitterness, clamour, wrath, slander, malice and obscene talk.  Anger does'nt keep good company.

If you're angry, deal with it.  Don't hold onto it.  Nip it in the bud.  Don't go to sleep on it.  Deal with it before the sun goes down.  At best anger is sin, and at worst it leads to even greater sins.  If we don't deal with these sins, they will poison our minds, and worse, they usually end up poisoning the minds of the people around us.  So that's why Jesus spoke strongly against anger.