There were several recipients of the Pour le Merite who were critical of the Nazis before and during WW2, among them Theo Osterkamp and Josef Jacobs, one of my distant relatives. To some degree, the PLM protected them from recriminations
but,unfortunately, this did not help your grand-father. You refer to the 20 July Plot by von Stauffenberg. Many were implicated (some no doubt unfairly in order to settle old scores),arrested,
tried by the People's (Kangaroo)Court,and executed
-while others committed suicide,or in Rommel's case, given a choice.
Your grandfather's death may be a little more mysterious. Alleged "conspirators" were still being arrested about that time, including some high-ranking ones such as Adm.Canaris,who was imprisoned and later executed. It appears your grandfather was murdered without any charges being
brought against him. A possible conclusion could be that he possessed knowledge of someone else's
involvement in the assassination plot and was killed to insure his silence.
However,finding any details will be difficult if not impossible to find, but perhaps you could explore these possibilities with your grandmother.
Good Luck and Best Wishes,