Three conditions must be met for a sin to be mortal. the CCC says…
1857 For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.”
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother.” The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
Your question seems to be trying to address the matter of gravity and doing so in a fairly broad way. I would suggest that you look at the “Golden Rule”, or the “Second Great commandment” as your guide in measuring gravity. We are to love others as we love ourselves. We are to treat others as we wish to be treated.
People who fight for sport go into the ring expecting to be treated by the rules and expectations of the ring. One who debates likewise expects their opponent to act within the proper parameters of the debate setting.
The above are formal settings. Outside of a formal setting, it can become a bit more difficult…but ultimately, when we are treating another in a way that we do not wish to be treated…we’ve moved into sin…
Once we recognize this, we have “knowledge” (sufficient to know it is a sin), and whether we continue in this becomes a choice (deliberate consent). Thus two of the three conditions of mortal sin are present.
As to the gravity of the matter…This is a bit of a judgement call. Your question is too broad to give any sort of definitive answer. The best guidance I can give is this.
When someone treats you in in a way you do not wish to be treated (through arguing or fighting or threats of fighting), how does it make you feel?
Are you merely “stung” by the treatment, or does it make you want to punish or even cut off contact with the other person?
If stung…then the matter is probably venial.
If deeper - to the point of cutting off contact with the other person - then the matter is more grave.
I know the above is not very precise. This is because every case is slightly different. If you have any concern over a given situation, it is best to confess it and to get the advice of a good confessor.
Peace
James