Believable, given a good reason.
For example....what if he is desperate? He knows something the others do not. The Army of Light is not just slow in being gathered....it doesn't exist. The Naaru have been defeated by the Legion, the few races they had managed to gather have been destroyed and the Naarus we see on Outland and with the Exarch are all that remain.
Desperation can lead to desperate acts that normally wouldn't be contemplated.
And - I think it would be an interesting twist.....**IF** Blizzard built it up properly, and dropped the odd hint about something being wrong, Velen hiding a secret and so on. Even if this were to happen....this woudl not be the right time for it. They totally messed up Benedictus' story
EJL