OK, that's half of it but the other half surely is empathising with the feelings of the victim (or scaree ?). He doesn't know how much (if any) real danger he is in. In that moment he is acting in self defence. The clown is causing a big emotion (fear) and shouldn't be surprised if this gives...
The bloke arrested in Norwich was 30 years old ! (not a typo...actually 30 years old !!)
http://www.edp24.co.uk/news/crime/man_dressed_as_clown_arrested_in_norwich_after_chasing_woman_through_eaton_park_1_4729254
what sort of a grown up gets their kicks from scaring people in this way ...