Seriously wth do you even want?
I could go on for hours speculating why he armed his ships if he didn't expect humans. I can have a gander at my own asshole and say 'Well, he didn't expect humans but it'd be foolhardy to sail unknown waters unarmed. There could be fucking Krakens for all I know.' which would probably be the correct assumption.
But in the end there is one reason:
Blizzard left tedious details out. The end. You can sleep at night again.