Option 1

Have a serious, calm conversation with your roommate, explaining the severity of your safety concerns and proposing a clear plan for ensuring the door is always locked, perhaps even suggesting a written agreement.

Option 2

Contact your landlord or building management to report your roommate's negligence, requesting that they intervene and enforce security rules, possibly leading to a formal warning or eviction.

Option 3

Start locking the door yourself every time you notice it's unlocked, and consider moving out quietly without confronting your roommate directly about the issue.