He may have been changed by what he saw in the Time War- after all, that was why he hid at the end of the universe.
On a narrative level, the Master is in some ways a darker version of the current Doctor. The joviality and hyperactive mania hiding a hard core of sociopathic cruelty reflects similar tendencies in the Tenth Doctor. Also, the Doctor needed to be shown where his god complex would lead, if he didn't reign it in, hence the Master's line "I'm a Time Lord. I have that right." (Unfortunately, it didn't stick.)
Also, he'd been listening to 'the drumming' inside his head for longer