Honestly, relax - he's saying immature things that he hasn't thought about for a millisecond - because he's five!
True, if he was fifteen and saying these things, then worry. But at five? Mine talks about killing people - he thinks they just get up and carry on walking like in a computer game. I am not in the least bit worried that he actually has murderous tendencies because when I look at his personality in total, he is happy, friendly, and wants everyone to join in. He doesn't exclude or hurt people, which indicates to me that the talk about "killing" is just a game to him. He's been asking lots of questions about what happens when someone dies as well so I think it's part of figuring out life for them.
"Get rid of" he could have heard in the context of old food being thrown away, clothes he has outgrown, unwanted components in a game (lego pieces not needed for his model, for example), "getting rid" of a charity chugger or door to door salesman - it's really not literal. He just means that he didn't want to play that game with her and he wanted her to leave him alone - absolutely nothing to do with dumping her because to him the idea of kissing is nothing about dating or love or relationships in that way, he just won't have the concept of it. Instead of making it into a gender-related discussion (at his age that's just another big marker for "girls and boys are different species" anyway which you don't want to encourage) it could be helpful to talk about nice/kind ways of asking somebody to stop doing something which is bothering you and how to deal with it if they won't stop.
I think it could just have easily been "Tom kept putting sand down my back and I didn't want him to but I got rid of him, I played with Ben instead, he doesn't put sand in people's clothes."
It doesn't mean he's taken Tom's head off with a spade and buried him in the sandpit 
it just means he managed to persuade him to leave him alone, or he got away, and went to play with somebody else instead.