Thank you for reporting! While the expression يلقي بظلاله على is used in journalism, it obviously doesn't work in every context, as in the example you provided and the one provided in our dictionary. While يفسد does seem to be a bit "too strong", as elroy pointed out, especially in the context of death, a translator can always find the best translation possible. For example, one suggestion for translating "Mark's death has cast a shadow over the whole event" is:
خيّم جوّ من الحزن على المناسبة لوفاة مارك.
The changes will be visible online after our next update, within a few months.