He's 4.4 now, it started low-level when he first started nursery which was almost exactly a year ago, but the physical aggression got really bad maybe 6 months or so ago? I kept assuming it was a phase and trying to manage it "gently" but this really wasn't working, when DP came over to visit (he works abroad) he was really shocked by the way DS was being with me and we ended up working out a plan - DS now gets a warning and if he continues then I put him in his room and it's helped a lot, but when I first did this he'd really pull out all the stops, he scratched me so badly I had a mark for about two weeks, and used to pull my hair really hard and try to bite/kick me. It's hard because he's not an aggressive child - but he was really aggressive with me!
I don't think that the smirking etc is anything to be concerned about. They really don't understand about killing, death etc, but they can understand enough that it's a bit of a taboo so they think it's funny in the same way as poo and bums and willies are funny (oh, we had a huge phase of this as well which seems to have disappeared with ignoring it and telling him nobody wanted to talk to him until he put his willy/bum away) - it's just because they can tell it's a bit "naughty" to talk about. If you explain it to him in a matter of fact way then it will probably come to seem less funny/naughty to him, but obviously it's tricky to explain what death is without scaring them! DS keeps "killing" things in games etc but then saying it's okay because it came back, even though we've talked about it he doesn't really understand I don't think.
But do answer his question. "Yes if you stepped on the guinea pig it would hurt the guinea pig a lot and it probably would kill it, and we would all be very sad." I probably would put the GP away just in case he thought you were giving him permission to do it! Another good technique is to ask him what he thinks would happen if he stepped on the guinea pig and whether it would be a nice or a nasty thing to do. You can get him thinking by thinking about what would happen if somebody stepped on his foot etc. And then talk about what would happen if the guinea pig died etc.
Sometimes when you're talking about stuff they don't appear to take it in either, but they do. The other week DS told me that a lady and a lady can't get married, because that's silly. I told him they can actually, if they want to, and he laughed as though I was making a big joke and kept saying "No mummy, only a man can marry a lady!" - I was really worried and even went through facebook to find a friend-of-a-friend's wedding photos to show him! He laughed through the whole explanation and I gave up but felt quite bad that I had failed to expose him to the concept of gay relationships
Anyway, later that day he came up to me and matter of factly said "Don't worry mummy, I know now that a lady can marry a lady, and a man can marry a man, or a lady can marry a man. Hey, this orange could marry this apple! Or a banana!"
I was a bit, er, that's not quite how it works, but sure. I was secretly pretty glad that it had gone in even though he was pretending not to listen at the time!