He doesn't like Williams. It's an immediate, and obviously mutual, feeling. Mostly, it's because he asks too many questions, interrupts and generally gets up in Nick's face at every opportunity. Part of the dislike is simply because Steve obviously does like and trust the guy.
Williams is a civilian; nothing more than a loud mouthed, opinionated, obnoxious detective with an overinflated ego. Steve isn't supposed to listen to someone like that over a former SEAL buddy. Steve's trust was one of the things that Nick was counting on when he found out that McGarrett was running a task force in Hawaii.
It's made things complicated when they should have been easy. McGarrett's a real smart guy, but his blind spot has always been loyalty. He's always given his trust, unreservedly, once he felt that someone has earned it, and he has always expected everyone else to do the same. Despite his years in the service, Steve's just never quite accepted that the world doesn't work the way he thinks it does. Nick had expected to be able to use that and the reminder of his and Steve's former friendship sand the things they've been through together to distract Steve. By the time McGarrett finally saw beyond the past and assumed shared objectives, it would have all been over and Nick would have been long gone.
But every time he tries to subtly deflect Steve's investigation and interest, Williams is there, like he and Steve are joined at the hip or something, butting in, all sarcasm and swagger and Nick doesn't know how Steve hasn't shot the guy yet. He would have, last time Nick met him.
Instead, he not only tolerates Williams, he's made him his partner and he actually seems to like him. More importantly, he listens to him. Steve paying attention to Williams wouldn't be such a problem though, if Williams weren't so suspicious. He doesn't say anything, but Nick can read the body language, the tone of voice and the way he looks at Nick.
That, and the fact that the detective is, despite appearances, pretty smart himself, makes him more of a threat than Nick expected. He still doesn't understand why Steve pays him so much attention though.
It's only later, when he catches Steve watching Williams that Nick starts to wonder. The expression on Steve's face, is part amusement, part exasperation and a whole lot of something that looks very like affection. It surprises Nick so much that he almost doesn't recognise the emotion at first. He remembers that Steve could be friendly enough, up for a beer or a night out in whatever shit hole town they were in at the time. But Steve was the guy that never really got close to the people he worked with, no matter how many beers he shared, or how many life and death situations he was in with them. There was always the sense that he was holding something back.
There's nothing like that reserve when he's with this team of his, and especially Williams. Steve seems to smile more, a lot of the hard, sharp edges that Nick remembers seem to have been smoothed out. And Nick discovers that Steve looks at his partner a lot, and there's always something soft and fond in the glances, as if the guy has somehow managed to dazzle Steve into thinking he's something special.
Once or twice, Nick's sure he's seen something other than just friendship in those looks, and it's not just Steve either. Williams does his share of looking too, he's just more subtle and a little harder to read than Steve is these days. It's something that Nick isn't surprised to see in an unstable, antagonistic cop with an attitude problem, but he'd thought better of Steve.
For a short time, right at the start, when he'd first found out that Steve was in Hawaii, he'd given the idea of trying to bring Steve onto the team some thought. He can see now though that it was a flawed idea. Still, it doesn't give Nick any pleasure to think that he might have to put Steve down in order to complete his own mission. But he's getting paid to do a job and if he has to, he has no problem with taking Steve and his asshole partner out in order to get it done.