This is something I've been wondering about for awhile. Why did Sleekwhisker just up and disappear?
To me, I loved Sleekwhisker's character. She's one of the only female villains, even if she's an extremely minor one. I feel like she had a ton of potential to carry on Darktail's legacy, or try to finish what he started. I liked how the Erin's brought her character back after Darktail's death, but then just dropped it all and had her run off with whatever was left of the Kin.
Similar to Darktail, Sleekwhisker was a manipulator. We didn't get to see much of it, but she did manage to convince Rowanstar she had changed and wanted to return to ShadowClan, only to betray him and take his last life. She definitely could've been a major villain, and a good one too, if she had the chance.
On the other hand, I also prefer that the Erin's didn't just kill her, instead letting her escape. If she had returned to be the next villain, she definitely would've ended up dying. So I think it's cool that she's still out there.
From what we've seen she's certainly capable of being cold hearted and merciless. Helping drown and kill many cats, including her best friend Needletail. It was even hinted that Dawnpelt, her mother, had been drowned, most likely meaning she helped with her own mothers demise.
Over all I think she was an interesting character and could've become more complex and a major villain if she had the chance.