Why is suicide considered a mortal sin in Catholicism?

The taking of any life is a mortal sin whether it be another or oneself. However we do not know what or where the person committing suicide soul is at or what is in the mind of that person. Only God know this and He will be the one who decides the offense. No one can judge another persons behavior. BTW murder is in the bible, in the old and new testament. God bless you.