Does he look sad to anyone else?
Okay, so he was the lamest villain ever, and he even had a lame villain song, and if I come close to shedding any sympathy for him I am going to have to pretend I didn't see him wreck Sunny's house because that is unforgivable.
But
Sprout is actually pretty interesting. He is a walking pile of complexes. He has a clearly unhealthy relationship with his mother. She didn't let him have that cupcake, and a villain was born. She also doesn't like him wandering around without permission, which is unusual by MLP parenting standards. In no gen have we ever seen them keep a strict watch over their foals, but I can tell she's different.
We learn right away that young Sprout wants to be sheriff someday, and he might have put some effort into pursuing that goal because he made it into Maretime Bay PD. Or, given the level of effort he puts into anything, I'm guessing his mother pulled some strings to get him that job. I think that's what's going on with him. He feels humiliated that he didn't become sheriff. And the way he got the job comes with an added humiliation hanging over his head.
And so he's just the deputy, doing all the stupid jobs like rescuing cats, still never in charge of anything meaningful, always playing second fiddle to stupid sexy Hitch.
And then he gets to be sheriff, and he changes. I don't think he goes Emperor too fast. Yes he's incredibly lazy, but he's lazy because he's never been in charge, maybe even on a personal level. He doesn't just get drunk on power here. He gets drunk on sudden motivation.
Everyone says this movie doesn't have any twists, but it kinda does. Or, not having a twist is the twist. What if Sprout's mother was a twist villain? What if she was the brains behind the whole thing? She's the one who made Sprout realize he's acting sheriff. She stood to benefit from that. Think how much Canterlogic stood to benefit from that. It's not hard to imagine that being the plot of the movie.
But no, that didn't happen. She was not trying to do that at all. She lost control of a monster she didn't plan on using in the first place. She just thought he'd be the cutest sheriff for a day and everything would go on as normal. She was as surprised as anyone else that Sprout was suddenly showing some motivation. She didn't know he had it in him.