For the first part,
1. "Many people get killed during accidents."
- This sentence suggests that the deaths occur in the midst of accidents, emphasizing the timing of the fatalities.
2. "Many people get killed in accidents."
- This sentence indicates that the deaths occur as a result of accidents, without specifying exactly when during the accidents they occur.
Both expressions are grammatically correct however, they convey slightly different meanings, and the choice between them depends on the specific emphasis you want to convey. However, "Many people get killed in accidents." is the more commonly used and straightforward phrasing to communicate that accidents are the cause of the fatalities.
For the next part,
Both "die of diseases" and "die from diseases" are considered correct in standard English, and they can be used interchangeably.