This dream seems to reflect a mix of fear, anxiety, and perhaps a sense of vulnerability. Here are some interpretations of the key elements:

1. Serial Killer: The presence of a serial killer can symbolize feelings of powerlessness or fear of something threatening in your waking life. It may represent a situation or emotion that feels out of control.
2. Influence of a Friend: Your friend transforming into the killer could indicate concerns about how external influences (like media or peers) can change people or situations. It might reflect worries about someone close to you adopting negative behaviors or attitudes.
3. Childhood Friends: The appearance of childhood friends suggests a longing for safety and comfort from your past. It could imply that you are seeking support or reassurance during stressful times.
4. Caution and Fear: Your cautious approach to entering your apartment indicates a heightened sense of awareness and fear of the unknown. This suggests that you may currently be facing a situation in life where you feel the need to be vigilant or protective.
5. Calling for Help: The act of wanting to call 911 signifies a desire for safety and assistance. It shows that you are aware of the dangers around you and are seeking solutions to overcome them.

Overall, this dream may be a manifestation of your subconscious fears and anxieties about change, external influences, and the need for safety. Consider reflecting on any current challenges you are facing and whether there are aspects of your life that feel threatening or out of your control. It might be helpful to talk to someone you trust about your feelings to gain further clarity and support.