As I implied, this character is completely repugnant. Castle finds him at the morgue where he is sexually harassing (with unwanted verbal advances) a hitherto unseen medical examiner, probably because different MEs are assigned to different units/precincts/whatever, but I'd prefer to think that it's because if he pulled that shit on Lanie, she would dissect his gonads and hand them to him in an evidence bag.
Anyway, for reasons best known to himself, Castle is intrigued by the rogue lawman and arranges a ride-around with him (that's where the barter of the brown coat comes in), takes Slaughter back to the 12th Precinct where he promptly checks out Beckett's ass. (We know this because the camera temporarily assumes his POV *sigh*). Unfortunately, she doesn't seem to notice, nor does Castle or anybody else, so nobody puts him in his place.
At this point, I was just longing for someone to hit this guy. Not that I condone the use of violence as conflict resolution in real life, but within the escapism of fiction, yes, absolutely: cads, oglers and other assorted assholes should be punched early and often. So I was just sort of chanting, like a mantra, "I really want someone to hit this guy. I don't care who but somebody, please, hit this guy!" And then, right before the second commercial break, it actually happened! And it was awesome!
What happened was that Slaughter started saying dirty things about Castle's daughter, Alexis (who's now an intern at the ME's office). He didn't know she was Castle's daughter, of course, but he didn't seem to care when Castle informed him of this fact, so Castle hit him. I actually literally cheered out loud. Very loud. I imagine that the neighbors didn't appreciate it and I'm sorry for that, but it was the best catharsis I have had in a while.
Unfortunately, it all rather went downhill from there. Castle didn't stop riding around with Slaughter (although I prefer to think that it was so he could protect Alexis from him if they had to go back to the morgue). Slaughter then rather made Castle his bitch and Beckett didn't even get to take her turn at hitting him (although she did get get to pull her whole staredown, intimidation bit on him, which is always quite fun and usually satisfying).
The next morning there were some some pop culture commentators that I watch sometimes complaining about the lack of (obvious?) Firefly references in the episode, and they were saying, (with regard to Slaughter basically making Castle his bitch), "C'mon! Captain Mal wouldn't have stood for that!" And I was like, "Dude, that is not Mal, that is Castle! He can't suddenly turn into Mal (except on Halloween) because that would just confuse the Castle fans who have never seen Firefly/Serenity, like my parents."
With that said, if we must compare Castle characters to Firefly characters...well, I don't like Jayne either, but at least he was occasionally funny and in the brief time that we knew him, he would occasionally show glimpses of having a deeper and more complex character. Slaughter was just shallow and unfunny and horrible. I may have read that his character was going to have a multi-episode arc, but I might be wrong about that. I hope that I am, because I never want to see that character again.
***MAJOR spoiler below***
And speaking of underdeveloped characters, why the HELL did Captain Gates allow Slaughter to just wander around her precinct? Is Castle's connection to the mayor really THAT powerful? I mean come on, if there were ever a character that "Iron Gates" should have barred from her precinct, it was this douchebag.
I'm done now.