Actually he did try to murder the baby once.
During the later seasons when he had Scrupel as an assistant, in one episode Gargamel admits to the kid how he created Smurfette and how much it blew chunks that she turned good/against him. Scrupel innocently asks why he can't just change her back, and Gargamel CLEARLY never thought to try based on his reaction.
So he finds out he CAN change her back with his own magic, the rest of the episode if her turning back into the black-haired Smurfette from her first appearance (she somehow had a fucking blond wig in her house to hide this from the other smurfs) and cue a whole bunch of bullshit where she tricks all the smurfs into getting captured by Gargamel.
So back at Gargamel's castle he's getting ready to cook them all, with Smurfette and Scrupel by his side, decides to start with Baby and despite being evil again, the fact that he immediately tries to kill the infant triggers Smurfette who turns good again and fucks everything up.
So I guess after that he learned his goddamn lesson because that's the only instance I can think of when he tried.