Stalin killed Trotsky because he was a political rival, in fact the one able of menace Stalin’s position. Jew or not jew was not a important issue, and they colaborated in Russian Civil War
Another thing: nor Azaña nor Negrin were commies. Azaña’s party, Republican Left, was burguese (maybe near to social-democracy). Negrin was socialist form the PSOE (the current ruling party in Spain now in 2009 by the way), he was not a commie. Even if PSOE was more left wing than now, they were not PCE (the commies), it was a difference and people recogniced it
Now if you talk of Ibarruri, Juan Díaz or Carrillo, they were commies. Carrillo is still alive by the way and had a important role in Spanish transition to democracy
And Hitler could not do a calmed strat. The guy was too emotional and dogmatic, unable of doing autocritic, even at inner level. Stalin was as cruel and sadic as Hitler, but Stalin was more open to autocritic at inner level and was less emotional, able of doing long term plans. He was dogmatic, but less than Hitler: Stalin couldn’t see what was Hitler planning but when he saw soviets could lose instead of doing as Hitler (babbling about how the Reich could not be beaten), Stalin changed some of his views and changed his strat
About no SCW, that would be indeed the better for Spain and for the world, for obvious reason (no destruction of Spain and no training for german army). In fact, even with rebelion, SCW was not a absolute must: if goverment (or Franco) would manage decide the situation early weeks instead of getting a stalemate, most of damage would be canceled (as in 1934). In that case you can bet for a neutral Spain, because it would be so difficult of invade as Switzerland