The scenes depicting the heyday of Fez’s grandmother’s reign were swiftly followed by ones capturing Fez and Ashtray’s current business dealings. At one point, Mouse (Meeko Gattuso) and his associate, Custer, paid them a visit to inquire about a recent raid at Fez and Ashtray’s home.
Fez heroically soldiered through the inquisition — until Ashtray walked in with a hammer in hand. He then hit Mouse in the head, killing him. He also injured Custer. While there’s no clear explanation from Ashtray, we can only assume that he was just trying to protect Fez. He’s extremely young throughout the show — noticeably younger than the rest of the characters — and was likely unable to keep his cool, scared of the potential for Mouse and Custer to seriously hurt Fez.
Source: https://www.distractify.com/p/why-did-ashtray-kill-mouse
