While Polish communists weren't the nicest or the smartest of people, they did have some brains; executing Wałęsa would have made him a martyr and only stiffen Polish opposition not to mention destroyed any Polish chance to make business with Western countries (to which Poland owed a lot of money); besides Polish communist leaders from 1980s were not Stalinist and the last thing they wanted was a big scale bloodshed.
However, in Polish United Workers' Party (Polish communist party) there was a group of hardliners, later known as "beton" (concrete); they actually were interested in Soviet intervention, which Jaruzelski would have prefered to avoid. Should they have taken over, say, in 1981, the martial law in Poland might have ended with much more casualties, Wałęsa included. And then teh Soviets migth have had to intervene (not that they were eager to do so).