It happens on the TV!
In all seriousness, I think an instant infatuation is one thing and although I claim no expertise I would suggest it does happen. What's scary in this case is the acting on it and doing something. In this case it seems like he's taken an immediate liking to her and totally lost touch with reality, for whatever reason. Knowing nothing about the accused or the crime makes it impossible to say, but I honestly believe anyone is capable of anything if the circumstances, motivation and opportunity are right.
Wow. I'm certainly not disputing what you're saying as I'm not remotely qualified to, but that description suggests something very instinctive and primitive lurking in the darkest corners of the human mind.
I don't think I'm alone in having had the odd thought of "I wouldn't mind a bit of that" - I'm pretty sure that the vast majority of post-pubescent individuals are prone to such thoughts, regardless of gender or sexual orientation. I say that as a married man who has never been unfaithful in his life.
You'd just like to think that in most healthy minded individuals, a sense of moral decency kicks-in. In certain scenarios, of course some people might seek to get what they want through 'legitimate' means (e.g. chatting the person up). I've always imagined that the tiny, tiny proportion of people capable of trying to fulfil their desires through force would have had to have harboured 'bad thoughts' for some time before they eventually act on them.
That said, I used to work with a chap who was eventually sacked following several episodes of deeply concerning, inappropriate behaviour towards women during staff nights out. You'd never have thought him capable of things like that during working hours - nicest bloke in the world. I'm a big believer that drink and drugs, when consumed in sufficient quantity, can strip away a person's 'front', revealing any personality flaws they may have. Some people are angry drunks, some people are emotional while others are pervy. It's not the drink that does it, it has to exist within the individual in the first place - the veneer is simply removed.
Anyway, I'm not suggesting that drink or drugs were a factor - I don't like speculating on tragic matters like this. It's just such a head scratcher.