My spouse says they want to die or kill themselves during quarrels or when under stress. What should I do?

Take it seriously every time, even if said in anger; make sure they are not alone, remove dangerous items, and get help immediately.
Allah forbids killing oneself and says He is Most Merciful to us (an-Nisa' 4:29), and preserving life is among the main aims of the Shariah (al-Ma'idah 5:32). Words like these are often a sign of deep emotional pain, not mere threats. Steps: listen without judging; ask directly whether they have a plan; do not promise to keep it secret; call Befrienders KL 03-7627 2929 or the HEAL line 15555, or take them to a hospital emergency department; if the danger is immediate, call 999. After the crisis, get ongoing psychiatric treatment or counselling. If such threats are used repeatedly to control you, get support for yourself as well. See Mental health.