Jan, it doesn't matter that Glady would never have friscaseed Mr. Fusspot. What's important here is that both Spike and Moist knew of the golem's jealous feelings and, because anything that can happen on the Discworld usually does, wrongfully assumed, after seeing Mr. Fusspot's collar on the ground, that her jealousy might manifest itself in poochcide. There's precedence for golems becoming killers when they don't understand the words in their heads--that's a central theme of Feet of Clay.
When they see Gladys in the kitchen staring at an overflowing pot their worst instincts kick in and they think it's possible, just possible, that Glady might have committed this foul deed. Spike tells her to give her the ladle. Gladys at first refuses--A GOLEM REFUSES A PERFECLY REASONABLE ORDER! Not because Gladys feels that Spike is questioning the quality of her work, but because she is still harboring resentments against Spike for stealing 'her man.' In the end Spike has to use her Golem Voice to get Gladys to obey her. And even though the doggie is safe, it's still clear that Gladys was still distracted and upset by Moist's rejection because she either forgot or failed to follow Peggy's order to remove the sheep's head from the pot once it thickened. (yes, intentional pune here)
It doesn't matter where Gladys's 'feelings' came from or what the golem is made of or what her job is. She is acting just like a spurned suitor. And if you think that she's nothing more than a matronly housekeeper, how do you explain her offer to give Moist a backrub? No housekeeper would ever do this, but is there a single woman who hasn't been grossed out by the offer of a backrub by some sleazy coworker she barely knows? If you tried this sort of thing in an American company, you'd risk getting your arse tossed out on a sling.