Lets start at the beginning.
Evil is something which contradicts the Divine mind. For instance, killing oneself is objectively evil. That is, the act itself is seen to contradict the will of God, which is that we should love God.
Catechism 2281 “Suicide is contrary to love for the living God.”
Now, we need to access if this is a serious matter, which we call a “grave” matter. Now, I think you can grasp yourself that this is a serious, or grave matter, but just to be sure what the Church teaches…
Catechism 2281"It is gravely contrary to the just love of self"
In other words, God wants us to love ourselves, and this is “gravely” contrary to that will.
So, now we can say that suicide is a grave matter.
Now, a mortal sin is grave matter, with deliberate consent, and full knowledge.
Catechism 1857 “Mortal sin is a sin whose object is a grave matter and which is also committed with full knowledge and deliberate consent”
OK, now we know we are dealing with a grave matter, but to make it a mortal sin we also need “deliberate consent” and “full knowledge”.
We understand these days that most of the people who commit suicide are mentally ill. For instance, scizophrenia, and depression are often the key causes.
In such a case there is a question over someones deliberate consent for a start. Often people with mental illnesses are compelled to act in ways which often bizarre, and sadly, some times even take their own life.Even if this is only a partial removal of their consent, it stops being deliberate. They have restricted freedom because they are not fully in control of their actions.
Secondly, in regards to full knowledge one must not have a distorted view of reality, such as someone who is having delusions, or someone who see’s the world in depression. Such people may not have an accurate view of reality, and so they do not have full knowledge.
For this reason the Church teaches…
Catechism 2282 “Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide”
So, the answer to your question is, it may be, but we hope it is not, and so we should commit that person to God’s loving arms. This is because we have no idea on the deliberate consent, or the full knowledge issues.
As the Church teaches
Catechism 1861: “However, although we can judge that an act is in itself a grave offense, we must entrust judgement of persons to the justice and mercy of God.”
I hope this answers your concerns.