Catechism:
"2302 By recalling the commandment, “You shall not kill,” our Lord asked for peace of heart and denounced murderous anger and hatred as immoral.
Anger is a desire for revenge. “To desire vengeance in order to do evil to someone who should be punished is illicit,” but it is praiseworthy to impose restitution “to correct vices and maintain justice.” If anger reaches the point of a deliberate desire to kill or seriously wound a neighbor, it is gravely against charity; it is a mortal sin. The Lord says, “Everyone who is angry with his brother shall be liable to judgment.”"
The Catechism quote gives the basics above (the CCC of course is not meant to be exhaustive)…(there is even “Just Anger” like that of Jesus in the Temple!)
…other things one could note is when it leads to serious injustice or serious scandal …or hatred of God etc or to consenting to such…
Also In terms of immoderate outbursts…such can often be venial sin…though if a person were to deliberately go into such a rage that it they can be considered to have lost their reason…then it is a different story.
I imagine more could be said …
But remember too…that especially with outbursts…even if there is grave matter…one must also have the knowledge and deliberate consent…
As to mortal sin:
Compendium --issued by Pope Benedict XVI
- When does one commit a mortal sin?
1855-1861
1874
One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.
- When does one commit a venial sin?
1862-1864
1875
One commits a venial sin, which is essentially different from a mortal sin, when the matter involved is less serious or, even if it is grave, when full knowledge or complete consent are absent. Venial sin does not break the covenant with God but it weakens charity and manifests a disordered affection for created goods. It impedes the progress of a soul in the exercise of the virtues and in the practice of moral good. It merits temporal punishment which purifies.