Very sad to hear this. I can't imagine how terrifying this was for any passengers in the same bit of the train, and for the staff, for that matter. It's almost worse that the person made off at Clandon, as there aren't going to have been an awful lot of resources immediately available to track them down over what could be a relatively large bit of mixed countryside and suburbia.
From a purely "commentary" perspective... whilst the local small towns and villages may be almost entirely peaceful, and have a lot of well-off commuters, some rather "interesting" characters board these trains at off-peak times. Years ago, Clandon was the first place I came across drug dealing on the railway. Some of the problems do remain. Guildford, too, has some troubled souls, despite being a bustling county town.