I'm not so convinced that it's going to be Sylar who's going to kill Claire's pops. I'm thinking it might actually be West when he eventually sees him. Maybe he remembers what they did to him and that sends him through the roof. I don't remember the picture that accurately, but did it look like it was Claire on the background kissing someone? Did she look distressed? Maybe West will kill HRG and seek understanding or something from Claire, who's like him and all that. Yeah, I'm not convinced at all that West and HRG know eachothers - the planting thing doesn't just sound right no matter how I try to think about it. HRG seems genuininely worried and even desperate to look after Claire - altough, it might be an act to make Claire realize how important it is to stay hidden, but I'm not holding my breath with that. But West and HRG? Nah.
And I do think that Sylar just walked like he said. Why not? It underlines the fact that he's now apparently powerless and vulnerable - and more hungry for power, if possible.
EDIT: Oh! I didn't realize it was Claire's car! Was it? It'd be pretty awesome way to set stuff in motion. This doesn't change my earlier opinion on who possibly might kill HRG. If anything, it makes me more certain it's not Sylar, since it's what everyone will think when they make it clear (to blind dorks like me) that Sylar has Claire's car.