It's not easy being a major league pitcher, particularly when you do something to raise the ire of the hometown fans. You and I make our fair share of mistakes every day, but we don't do it in front of tens of thousands of spectators, not to mention thousands or even millions more on television. And when we screw up, nobody is waiting to boo and jeer until they're hoarse. We're not going to get pelted with beer cups and peanut shells. The phone lines at the local talk radio station won't be burning up with impassioned calls for our dismissal. Most importantly, no one will mail us a fake prescription for cyanide, "to be taken daily".
Now, Doug Sisk was probably a frustrating pitcher to watch. A scan of his career record shows that the sinkerballer routinely walked more batters than he struck out. It's also possible that the supposed Mets fan in the anecdote wrote his clever missive in the midst of Sisk's brutal 1985 season. But even so, Doug couldn't have been bad enough to deserve death. The things that he did, he did very well: his sinker allowed him to keep the ball in the park, as he surrendered only 15 home runs in over 523 career innings. You'd almost certainly have to have a pretty thick skin to be a ballplayer. Even if 99.9% of the threats are empty, I know I would worry about the other 0.1%.