Rommel's suicide was a tragedy. It is still hotly debated as to whether in fact he was ever a member of the conspiracy, or even how much he knew. To me it is telling that the conspirators in touch with John Foster Dulles (Speidel I think, from memory) advised the allies that Rommel could not be counted on to support the coup.
he was killed on the basis he might have known about the plot and did nothing, the evidence that implicated him came from the torture induced confessions of Stulnagel and Hofacker, though his friendship with Kluge was also a prime reason for his death. by any standard the evidence leading to his death was flimsy, but in the context of post 20 July 1944, this was enough to get anyone killed