There are different theories as to what did kill Mozart. One of the arguments is tied to Vitamin D deficiency which was said he suffered from. He would sleep during the day and compose music at night. Another theory was that he was poisoned from arsenic.
Some also say he could've died from strep/rheumatic fever which was on an outbreak around the time he died. Researchers investigated Vienna death records during that time frame, and many died from this affliction.