The info I have is slightly but significantly different. Capt. Ganjoor initially refused to let the minister's bodyguard carry his gun onboard but relented later after much argument at the intervention of the PEN station manager who quoted a directive from MAS management allowing VIPs to have the weapons onboard.
The bodyguard was furious and became even more so after imbibing lots of alcohol........proceeded to the cockpit after threatening the cabin crew and the rest is history. However I have some doubts as to whether it was plain air rage or he and the minister had earlier planned something nefarious, especially the unwavering insistence to have his gun aboard. Coupled with the Captain's transmission that they were proceeding to S'pore after the hijack action; so this didn't seem a case of plain air rage.