So, according to the prophecy their son will rule the universe. But they just killed The Beast, an evil monster trying to take over the universe. So... their son finished the job that The Beast started? Wouldn't that make him just as evil as The Beast was?
Presumably, their son was going to do it in a more peaceful way. The narration certainly implies that his rule will be a good thing.
Also, galaxy, not universe. Hey, it's a difference of I don't know how many trillions of times the same amount of space!
Indeed, there's no need to assume that "rule" automatically includes "... with an iron fist". It's a fantasy movie, good kings who rule nobly and justly are behind every third happy ending.
Geese, sure. Pigs, sure. But why would Ergo apparently carry around a spell that would turn people into tigers, which would, if it actually worked, put him in a far worse situation than originally?
Yeah, he tried to use the first two as a Baleful Polymorph before they backfired, but consider this: You're a traveling wizard, and a local lord wants to hire you to defend against X threat. Just zap some of his soldiers into tigers and let them loose on your enemy. Likewise, Ergo might hire some locals to defend himself and zap them into tigers. It might also be he's an errant disciple who only learned to cast spells on himself (and so they always backfire when cast on others). In which case it made perfect sense to have the Tiger spell for self defense. Now, if he'd been cleverer (a dangerous thing for the likes of him) he could try to zap the middle guy in an enemy group into a tiger, and loudly shout "Now my new beast-slave, devour your former companions!" The rest of the group could be so scared and panicky, and the transformed bandit so freaked out and unable to talk, that a fight would likely break out and hopefully reducing their numbers. It would at the least cause enough of a diversion for him to run/fly away.
Ergo apparently only has the ability to transform himself: whether he doesn't realize that early on or just tries to bluff people is a little unclear. In any case, he can apparently use the same spell to achieve any particular form he chooses.