They made it appear that it was because he was hurt to kill a student but hiruzen never seemed to be a match for any of his students, he's all hype but only when compared to a genin or basic Jonin
Orochimaru probably would've wrecked him, he only really lost his arms because he was an. Arrogant prick and wanted to toy with his food, the edo's whereby nessecary because they weren't even impressive , they where just there to toy with his feelings.
The funny thing is he was more scared by Tsunades temper than he was when the reaper had his arms