1. Trump's comment was inappropriate.
However ...
2. Reiner spent a decade slandering Trump as a murderer, traitor, Racist, and White Supremacist. While not justified by a Christian POV (AFAIK, Trump is not and does not claim to be a Christian believer), after 10 years of being slandered, Trump's post is understandable and comparatively mild.
3. BTW, Reiner also, for 10 years, slandered Trump voters as Racists and White Supremacists, so I'm not deeply mourning Reiner's death, as sad as it was.